diff --git a/example/Douban/test.hs b/example/Douban/test.hs
--- a/example/Douban/test.hs
+++ b/example/Douban/test.hs
@@ -1,6 +1,8 @@
 {-# LANGUAGE FlexibleContexts  #-}
 {-# LANGUAGE OverloadedStrings #-}
 {-# LANGUAGE QuasiQuotes       #-}
+{-# LANGUAGE DeriveGeneric     #-}
+{-# LANGUAGE ScopedTypeVariables #-}
 
 {-
 
@@ -13,11 +15,9 @@
 module Main where
 
 import qualified Data.ByteString.Char8      as BS
-import qualified Data.ByteString.Lazy.Char8 as BSL
 import qualified Data.Text                  as T
 import qualified Data.Text.Encoding         as T
 import qualified Data.Text.Lazy.Encoding    as TL
-import qualified Data.Text.Lazy.IO          as TL
 import           Network.HTTP.Conduit
 import           URI.ByteString
 import           URI.ByteString.QQ
@@ -26,6 +26,17 @@
 
 import           Keys                       (doubanKey)
 
+import           GHC.Generics
+import           Data.Aeson
+import           Data.Aeson.Types
+
+data Errors =
+  SomeRandomError
+  deriving (Show, Eq, Generic)
+
+instance FromJSON Errors where
+  parseJSON = genericParseJSON defaultOptions { constructorTagModifier = camelTo2 '_', allNullaryToStringTag = True }
+
 main :: IO ()
 main = do
   BS.putStrLn $ serializeURIRef' $ authorizationUrl doubanKey
@@ -37,13 +48,14 @@
                   , ("client_secret", T.encodeUtf8 $ oauthClientSecret doubanKey)
                   ]
 
-  token <- doJSONPostRequest mgr doubanKey url (extraBody ++ body)
+  token :: OAuth2Result Errors OAuth2Token <- doJSONPostRequest mgr doubanKey url (extraBody ++ body)
   print token
   case token of
     Right r -> do
+      -- TODO: display Chinese character. (Text UTF-8 encodeing does not work, why?)
       uid <- authGetBS mgr (accessToken r) [uri|https://api.douban.com/v2/user/~me|]
-      TL.putStrLn $ either TL.decodeUtf8 TL.decodeUtf8 uid
-    Left l -> BSL.putStrLn l
+      putStrLn $ either (show :: OAuth2Error Errors -> String) (show . TL.decodeUtf8) uid
+    Left l -> print l
 
 sToBS :: String -> BS.ByteString
 sToBS = T.encodeUtf8 . T.pack
diff --git a/example/Dropbox/test.hs b/example/Dropbox/test.hs
--- a/example/Dropbox/test.hs
+++ b/example/Dropbox/test.hs
@@ -1,20 +1,30 @@
 {-# LANGUAGE FlexibleContexts  #-}
 {-# LANGUAGE OverloadedStrings #-}
+{-# LANGUAGE DeriveGeneric     #-}
 
 module Main where
 
 import qualified Data.ByteString.Char8      as BS
 import qualified Data.ByteString.Lazy.Char8 as BSL
 import qualified Data.Text                  as T
+import           Data.Aeson
+import           Data.Aeson.Types
 import           Network.HTTP.Conduit
 import qualified Network.HTTP.Types         as HT
 import           URI.ByteString
+import           GHC.Generics
 
 import           Network.OAuth.OAuth2
 
 import           Keys
 
+data Errors =
+  SomeRandomError
+  deriving (Show, Eq, Generic)
 
+instance FromJSON Errors where
+  parseJSON = genericParseJSON defaultOptions { constructorTagModifier = camelTo2 '_', allNullaryToStringTag = True }
+
 main :: IO ()
 main = do
     BS.putStrLn $ serializeURIRef' $ authorizationUrl dropboxKey
@@ -27,7 +37,7 @@
       Right at -> getSpaceUsage mgr (accessToken at) >>= print
       Left _   -> putStrLn "no access token found yet"
 
-getSpaceUsage :: Manager -> AccessToken -> IO (OAuth2Result BSL.ByteString)
+getSpaceUsage :: Manager -> AccessToken -> IO (OAuth2Result Errors BSL.ByteString)
 getSpaceUsage mgr token = do
   req <- parseRequest $ BS.unpack "https://api.dropboxapi.com/2/users/get_space_usage"
   authRequest req upReq mgr
diff --git a/example/Facebook/test.hs b/example/Facebook/test.hs
--- a/example/Facebook/test.hs
+++ b/example/Facebook/test.hs
@@ -2,6 +2,7 @@
 {-# LANGUAGE OverloadedStrings #-}
 {-# LANGUAGE QuasiQuotes       #-}
 {-# LANGUAGE TemplateHaskell   #-}
+{-# LANGUAGE DeriveGeneric     #-}
 
 {- Facebook example -}
 
@@ -10,7 +11,6 @@
 import           Keys                       (facebookKey)
 import           Network.OAuth.OAuth2
 
-import           Data.Aeson                 (FromJSON)
 import           Data.Aeson.TH              (defaultOptions, deriveJSON)
 import qualified Data.ByteString.Lazy.Char8 as BL
 import           Data.Text                  (Text)
@@ -21,6 +21,18 @@
 import           URI.ByteString
 import           URI.ByteString.QQ
 
+import           GHC.Generics
+import           Data.Aeson
+import           Data.Aeson.Types
+
+data Errors =
+  SomeRandomError
+  deriving (Show, Eq, Generic)
+
+instance FromJSON Errors where
+  parseJSON = genericParseJSON defaultOptions { constructorTagModifier = camelTo2 '_', allNullaryToStringTag = True }
+
+
 --------------------------------------------------
 
 data User = User { id    :: Text
@@ -44,7 +56,7 @@
                     , ("client_secret", T.encodeUtf8 $ oauthClientSecret facebookKey)
                     ]
     resp <- doJSONPostRequest mgr facebookKey url (body ++ extraBody)
-    case (resp :: OAuth2Result OAuth2Token) of
+    case (resp :: OAuth2Result Errors OAuth2Token) of
       Right token -> do
                      print token
                      userinfo mgr (accessToken token) >>= print
@@ -59,8 +71,8 @@
 facebookScope = [("scope", "user_about_me,email")]
 
 -- | Fetch user id and email.
-userinfo :: Manager -> AccessToken -> IO (OAuth2Result BL.ByteString)
+userinfo :: Manager -> AccessToken -> IO (OAuth2Result Errors BL.ByteString)
 userinfo mgr token = authGetBS mgr token    [uri|https://graph.facebook.com/me?fields=id,name,email|]
 
-userinfo' :: FromJSON User => Manager -> AccessToken -> IO (OAuth2Result User)
+userinfo' :: FromJSON User => Manager -> AccessToken -> IO (OAuth2Result Errors User)
 userinfo' mgr token = authGetJSON mgr token [uri|https://graph.facebook.com/me?fields=id,name,email|]
diff --git a/example/Fitbit/test.hs b/example/Fitbit/test.hs
--- a/example/Fitbit/test.hs
+++ b/example/Fitbit/test.hs
@@ -1,11 +1,12 @@
 {-# LANGUAGE FlexibleContexts  #-}
 {-# LANGUAGE OverloadedStrings #-}
 {-# LANGUAGE QuasiQuotes       #-}
+{-# LANGUAGE DeriveGeneric     #-}
+{-# LANGUAGE ScopedTypeVariables #-}
 
 module Main where
 
 import           Control.Monad            (mzero)
-import           Data.Aeson
 import qualified Data.ByteString          as B
 import qualified Data.ByteString.Lazy     as BL
 import           Data.Char                (chr)
@@ -23,6 +24,18 @@
 import           Keys                     (fitbitKey)
 import           Network.OAuth.OAuth2
 
+import           GHC.Generics
+import           Data.Aeson
+import           Data.Aeson.Types
+
+data Errors =
+  SomeRandomError
+  deriving (Show, Eq, Generic)
+
+instance FromJSON Errors where
+  parseJSON = genericParseJSON defaultOptions { constructorTagModifier = camelTo2 '_', allNullaryToStringTag = True }
+
+
 ------------------------------------------------------------------------------
 
 data FitbitUser = FitbitUser
@@ -78,7 +91,7 @@
 getApiCode request =
     case M.lookup "code" queryMap of
         Just code -> ExchangeToken $ T.decodeUtf8 code
-        Nothing   -> error "request doesn't include code"
+        Nothing   -> Prelude.error "request doesn't include code"
   where
     queryMap = convertQueryToMap $ queryString request
 
@@ -87,7 +100,7 @@
     result <- doJSONPostRequest mgr fitbitKey url $ body ++ [("state", state)]
     case result of
         Right token -> return token
-        Left e      -> error $ lazyBSToString e
+        Left (e :: OAuth2Error Errors) -> Prelude.error $ show e
   where
     (url, body) = accessTokenUrl fitbitKey code
 
@@ -96,7 +109,7 @@
     result <- authGetJSON mgr token [uri|https://api.fitbit.com/1/user/-/profile.json|]
     case result of
         Right user -> return user
-        Left e     -> error $ lazyBSToString e
+        Left (e :: OAuth2Error Errors) -> Prelude.error $ show e
 
 convertQueryToMap :: Query -> M.Map B.ByteString B.ByteString
 convertQueryToMap query =
diff --git a/example/Github/test.hs b/example/Github/test.hs
--- a/example/Github/test.hs
+++ b/example/Github/test.hs
@@ -1,13 +1,13 @@
 {-# LANGUAGE FlexibleContexts  #-}
 {-# LANGUAGE OverloadedStrings #-}
 {-# LANGUAGE QuasiQuotes       #-}
+{-# LANGUAGE DeriveGeneric     #-}
 
 -- | Github API: http://developer.github.com/v3/oauth/
 
 module Main where
 
 import           Control.Monad        (mzero)
-import           Data.Aeson
 import qualified Data.ByteString      as BS
 import           Data.Text            (Text)
 import qualified Data.Text            as T
@@ -17,28 +17,47 @@
 import           URI.ByteString.QQ
 
 import           Network.OAuth.OAuth2
+import qualified Network.OAuth.OAuth2.AuthorizationRequest as AR
 
 import           Keys
 
+import           GHC.Generics
+import           Data.Aeson
+import           Data.Aeson.Types
 
+data Errors =
+  SomeRandomError
+  deriving (Show, Eq, Generic)
+
+instance FromJSON Errors where
+  parseJSON = genericParseJSON defaultOptions { constructorTagModifier = camelTo2 '_', allNullaryToStringTag = True }
+
+
+
 main :: IO ()
 main = do
     let state = "testGithubApi"
+    mgr <- newManager tlsManagerSettings
+    putStrLn "Trying invalid token..."
+    failToken <- getToken state "invalidCode" mgr
+    print (failToken :: OAuth2Result AR.Errors OAuth2Token)
     print $ serializeURIRef' $ appendQueryParams [("state", state)] $ authorizationUrl githubKey
     putStrLn "visit the url and paste code here: "
     code <- getLine
-    mgr <- newManager tlsManagerSettings
-    let (url, body) = accessTokenUrl githubKey $ ExchangeToken $ T.pack code
-    token <- doJSONPostRequest mgr githubKey url (body ++ [("state", state)])
-    print (token :: OAuth2Result OAuth2Token)
+    token <- getToken state code mgr
+    print (token :: OAuth2Result AR.Errors OAuth2Token)
     case token of
       Right at -> userInfo mgr (accessToken at) >>= print
       Left _   -> putStrLn "no access token found yet"
 
+getToken :: FromJSON a => BS.ByteString -> String -> Manager -> IO (OAuth2Result AR.Errors a)
+getToken state code mgr = do
+    let (url, body) = accessTokenUrl githubKey $ ExchangeToken $ T.pack code
+    doJSONPostRequest mgr githubKey url (body ++ [("state", state)])
 
 -- | Test API: user
 --
-userInfo :: Manager -> AccessToken -> IO (OAuth2Result GithubUser)
+userInfo :: Manager -> AccessToken -> IO (OAuth2Result (OAuth2Error Errors) GithubUser)
 userInfo mgr token = authGetJSON mgr token [uri|https://api.github.com/user|]
 
 data GithubUser = GithubUser { gid   :: Integer
diff --git a/example/Google/test.hs b/example/Google/test.hs
--- a/example/Google/test.hs
+++ b/example/Google/test.hs
@@ -30,6 +30,13 @@
 import           Keys                          (googleKey)
 import           Network.OAuth.OAuth2
 
+data Errors =
+  SomeRandomError
+  deriving (Show, Eq, Generic)
+
+instance FromJSON Errors where
+  parseJSON = genericParseJSON defaultOptions { constructorTagModifier = camelTo2 '_', allNullaryToStringTag = True }
+
 --------------------------------------------------
 
 data Token = Token { issuedTo   :: Text
@@ -76,7 +83,7 @@
 offlineCase :: Manager -> IO ()
 offlineCase mgr = do
     BS.putStrLn $ serializeURIRef' $ appendQueryParams (googleScopeEmail ++ googleAccessOffline) $ authorizationUrl googleKey
-    putStrLn "visit the url and paste code here: "
+    putStrLn "offline mode: visit the url and paste code here: "
     code <- getLine
     (Right token) <- fetchAccessToken mgr googleKey $ ExchangeToken $ T.pack code
     f (accessToken token)
@@ -88,29 +95,34 @@
         Nothing -> putStrLn "Failed to fetch refresh token"
         Just tk -> do
             (Right token') <- fetchRefreshToken mgr googleKey tk
-            f token'
+            f (accessToken token')
             --validateToken accessToken >>= print
             --(validateToken' accessToken :: IO (OAuth2Result Token)) >>= print
     where f token = do
             print token
             validateToken mgr token >>= print
-            (validateToken' mgr token :: IO (OAuth2Result Token)) >>= print
+            (validateToken' mgr token :: IO (OAuth2Result (OAuth2Error Errors) Token)) >>= print
 
 normalCase :: Manager -> IO ()
 normalCase mgr = do
+    -- try an invalid token
+    putStr "Trying invalid token..."
+    validateToken mgr (AccessToken "invalid") >>= print
     BS.putStrLn $ serializeURIRef' $ appendQueryParams googleScopeUserInfo (authorizationUrl googleKey)
-    putStrLn "visit the url and paste code here: "
+    putStrLn "normal mode: visit the url and paste code here: "
     code <- fmap (ExchangeToken . T.pack) getLine
-    (Right token) <- fetchAccessToken mgr googleKey code
+    maybeToken <- fetchAccessToken mgr googleKey code
+    print maybeToken
+    (Right token) <- return maybeToken
     putStr "AccessToken: " >> print token
     -- get response in ByteString
     validateToken mgr (accessToken token) >>= print
     -- get response in JSON
-    (validateToken' mgr (accessToken token):: IO (OAuth2Result Token)) >>= print
+    (validateToken' mgr (accessToken token):: IO (OAuth2Result (OAuth2Error Errors) Token)) >>= print
     -- get response in ByteString
     userinfo mgr (accessToken token) >>= print
     -- get response in JSON
-    (userinfo' mgr (accessToken token) :: IO (OAuth2Result User)) >>= print
+    (userinfo' mgr (accessToken token) :: IO (OAuth2Result (OAuth2Error Errors) User)) >>= print
 
 --------------------------------------------------
 -- Google API
@@ -131,7 +143,7 @@
 -- | Token Validation
 validateToken :: Manager
                  -> AccessToken
-                 -> IO (OAuth2Result BL.ByteString)
+                 -> IO (OAuth2Result (OAuth2Error Errors) BL.ByteString)
 validateToken mgr token =
    authGetBS' mgr token url
    where url = [uri|https://www.googleapis.com/oauth2/v1/tokeninfo|]
@@ -139,7 +151,7 @@
 validateToken' :: FromJSON a
                   => Manager
                   -> AccessToken
-                  -> IO (OAuth2Result a)
+                  -> IO (OAuth2Result (OAuth2Error Errors) a)
 validateToken' mgr token = parseResponseJSON <$> validateToken mgr token
 
 -- | fetch user email.
@@ -147,11 +159,11 @@
 --
 userinfo :: Manager
             -> AccessToken
-            -> IO (OAuth2Result BL.ByteString)
+            -> IO (OAuth2Result (OAuth2Error Errors) BL.ByteString)
 userinfo mgr token = authGetBS mgr token [uri|https://www.googleapis.com/oauth2/v2/userinfo|]
 
 userinfo' :: FromJSON a
              => Manager
              -> AccessToken
-             -> IO (OAuth2Result a)
+             -> IO (OAuth2Result (OAuth2Error Errors) a)
 userinfo' mgr token = authGetJSON mgr token [uri|https://www.googleapis.com/oauth2/v2/userinfo|]
diff --git a/example/StackExchange/test.hs b/example/StackExchange/test.hs
--- a/example/StackExchange/test.hs
+++ b/example/StackExchange/test.hs
@@ -2,6 +2,7 @@
 {-# LANGUAGE FlexibleContexts  #-}
 {-# LANGUAGE OverloadedStrings #-}
 {-# LANGUAGE QuasiQuotes       #-}
+{-# LANGUAGE ScopedTypeVariables #-}
 
 -- | https://api.stackexchange.com/docs/authentication
 
@@ -54,7 +55,15 @@
 instance ToJSON SiteItem where
     toEncoding = genericToEncoding defaultOptions { fieldLabelModifier = camelTo2 '_' }
 
+data Errors =
+  SomeRandomError
+  deriving (Show, Eq, Generic)
 
+instance FromJSON Errors where
+  parseJSON = genericParseJSON defaultOptions { constructorTagModifier = camelTo2 '_', allNullaryToStringTag = True }
+
+
+
 main :: IO ()
 main = do
     BS.putStrLn $ serializeURIRef' $ authorizationUrl stackexchangeKey
@@ -75,10 +84,10 @@
     print token
     case token of
       Right at -> siteInfo mgr (accessToken at) >>= print
-      Left _   -> putStrLn "no access token found yet"
+      Left (_ :: OAuth2Error Errors) -> putStrLn "no access token found yet"
 
 -- | Test API: info
-siteInfo :: Manager -> AccessToken -> IO (OAuth2Result SiteInfo)
+siteInfo :: Manager -> AccessToken -> IO (OAuth2Result (OAuth2Error Errors) SiteInfo)
 siteInfo mgr token = authGetJSON mgr token [uri|https://api.stackexchange.com/2.2/info?site=stackoverflow|]
 
 sToBS :: String -> BS.ByteString
diff --git a/example/Weibo/test.hs b/example/Weibo/test.hs
--- a/example/Weibo/test.hs
+++ b/example/Weibo/test.hs
@@ -1,6 +1,7 @@
 {-# LANGUAGE FlexibleContexts  #-}
 {-# LANGUAGE OverloadedStrings #-}
 {-# LANGUAGE QuasiQuotes       #-}
+{-# LANGUAGE DeriveGeneric     #-}
 
 {-
 
@@ -22,7 +23,7 @@
 module Main where
 
 import qualified Data.ByteString            as BS
-import qualified Data.ByteString.Lazy.Char8 as BSL
+import qualified Data.ByteString.Lazy       as BSL
 import qualified Data.Text                  as T
 import qualified Data.Text.Encoding         as T
 import           Network.HTTP.Conduit
@@ -32,6 +33,18 @@
 
 import           Keys
 
+import           GHC.Generics
+import           Data.Aeson
+import           Data.Aeson.Types
+
+data Errors =
+  SomeRandomError
+  deriving (Show, Eq, Generic)
+
+instance FromJSON Errors where
+  parseJSON = genericParseJSON defaultOptions { constructorTagModifier = camelTo2 '_', allNullaryToStringTag = True }
+
+
 main :: IO ()
 main = do
        print $ serializeURIRef' $ authorizationUrl weiboKey
@@ -43,8 +56,8 @@
        case token of
          Right r -> do
                     uid <- authGetBS' mgr (accessToken r) [uri|https://api.weibo.com/2/account/get_uid.json|]
-                    print uid
-         Left l -> BSL.putStrLn l
+                    print (uid :: OAuth2Result (OAuth2Error Errors) BSL.ByteString)
+         Left l -> print l
 
 sToBS :: String -> BS.ByteString
 sToBS = T.encodeUtf8 . T.pack
diff --git a/hoauth2.cabal b/hoauth2.cabal
--- a/hoauth2.cabal
+++ b/hoauth2.cabal
@@ -1,6 +1,6 @@
 Name:                hoauth2
 -- http://wiki.haskell.org/Package_versioning_policy
-Version:             1.2.0
+Version:             1.3.0
 
 Synopsis:            Haskell OAuth2 authentication client
 
@@ -58,6 +58,8 @@
   Exposed-modules: Network.OAuth.OAuth2.HttpClient
                    Network.OAuth.OAuth2.Internal
                    Network.OAuth.OAuth2
+                   Network.OAuth.OAuth2.TokenRequest
+                   Network.OAuth.OAuth2.AuthorizationRequest
 
   Build-Depends: base                 >= 4     && < 5,
                  text                 >= 0.11  && < 1.3,
@@ -67,6 +69,7 @@
                  aeson                >= 0.11  && < 1.3,
                  unordered-containers >= 0.2.5,
                  uri-bytestring       >= 0.2.3.1 && < 0.3,
+                 uri-bytestring-aeson >= 0.1   && < 0.2,
                  microlens            >= 0.4.0 && < 0.5,
                  exceptions           >= 0.8.3 && < 0.9
 
diff --git a/src/Network/OAuth/OAuth2/AuthorizationRequest.hs b/src/Network/OAuth/OAuth2/AuthorizationRequest.hs
new file mode 100644
--- /dev/null
+++ b/src/Network/OAuth/OAuth2/AuthorizationRequest.hs
@@ -0,0 +1,24 @@
+{-# LANGUAGE DeriveGeneric              #-}
+
+module Network.OAuth.OAuth2.AuthorizationRequest where
+
+import           Data.Aeson
+import           Data.Aeson.Types
+import           GHC.Generics
+
+instance FromJSON Errors where
+  parseJSON = genericParseJSON defaultOptions { constructorTagModifier = camelTo2 '_', allNullaryToStringTag = True }
+instance ToJSON Errors where
+  toEncoding = genericToEncoding defaultOptions { constructorTagModifier = camelTo2 '_', allNullaryToStringTag = True }
+
+-- | Authorization Code Grant Error Responses https://tools.ietf.org/html/rfc6749#section-4.1.2.1
+-- Implicit Grant Error Responses https://tools.ietf.org/html/rfc6749#section-4.2.2.1
+data Errors =
+    InvalidRequest
+  | UnauthorizedClient
+  | AccessDenied
+  | UnsupportedResponseType
+  | InvalidScope
+  | ServerError
+  | TemporarilyUnavailable
+  deriving (Show, Eq, Generic)
diff --git a/src/Network/OAuth/OAuth2/HttpClient.hs b/src/Network/OAuth/OAuth2/HttpClient.hs
--- a/src/Network/OAuth/OAuth2/HttpClient.hs
+++ b/src/Network/OAuth/OAuth2/HttpClient.hs
@@ -37,53 +37,55 @@
 import           Network.HTTP.Types.URI        (parseQuery)
 import           Network.OAuth.OAuth2.Internal
 import           URI.ByteString
+import qualified Network.OAuth.OAuth2.TokenRequest as TR
 
 --------------------------------------------------
 -- * Token management
 --------------------------------------------------
 
 -- | Request (via POST method) "OAuth2 Token".
-fetchAccessToken :: Manager                          -- ^ HTTP connection manager
-                   -> OAuth2                         -- ^ OAuth Data
-                   -> ExchangeToken                  -- ^ OAuth 2 Tokens
-                   -> IO (OAuth2Result OAuth2Token)  -- ^ Access Token
+fetchAccessToken :: Manager                                   -- ^ HTTP connection manager
+                   -> OAuth2                                  -- ^ OAuth Data
+                   -> ExchangeToken                           -- ^ OAuth 2 Tokens
+                   -> IO (OAuth2Result TR.Errors OAuth2Token) -- ^ Access Token
 fetchAccessToken manager oa code = doFlexiblePostRequest manager oa uri body
                            where (uri, body) = accessTokenUrl oa code
 
 
--- | Request the "Refresh Token".
+-- | Request a new AccessToken with the Refresh Token.
+-- TODO: seems more approporate to rename to refreshAccessToken
 fetchRefreshToken :: Manager                         -- ^ HTTP connection manager.
                      -> OAuth2                       -- ^ OAuth context
                      -> RefreshToken                 -- ^ refresh token gained after authorization
-                     -> IO (OAuth2Result AccessToken)
+                     -> IO (OAuth2Result TR.Errors OAuth2Token)
 fetchRefreshToken manager oa token = doFlexiblePostRequest manager oa uri body
                               where (uri, body) = refreshAccessTokenUrl oa token
 
 
 -- | Conduct post request and return response as JSON.
-doJSONPostRequest :: FromJSON a
+doJSONPostRequest :: FromJSON err => FromJSON a
                   => Manager                             -- ^ HTTP connection manager.
                   -> OAuth2                              -- ^ OAuth options
                   -> URI                                 -- ^ The URL
                   -> PostBody                            -- ^ request body
-                  -> IO (OAuth2Result a)                 -- ^ Response as ByteString
+                  -> IO (OAuth2Result err a)             -- ^ Response as JSON
 doJSONPostRequest manager oa uri body = fmap parseResponseJSON (doSimplePostRequest manager oa uri body)
 
 -- | Conduct post request and return response as JSON or Query String.
-doFlexiblePostRequest :: FromJSON a
+doFlexiblePostRequest :: FromJSON err => FromJSON a
                          => Manager                             -- ^ HTTP connection manager.
                          -> OAuth2                              -- ^ OAuth options
                          -> URI                                 -- ^ The URL
                          -> PostBody                            -- ^ request body
-                         -> IO (OAuth2Result a)                 -- ^ Response as ByteString
+                         -> IO (OAuth2Result err a)             -- ^ Response as ByteString
 doFlexiblePostRequest manager oa uri body = fmap parseResponseFlexible (doSimplePostRequest manager oa uri body)
 
 -- | Conduct post request.
-doSimplePostRequest :: Manager                              -- ^ HTTP connection manager.
-                       -> OAuth2                            -- ^ OAuth options
-                       -> URI                               -- ^ URL
-                       -> PostBody                          -- ^ Request body.
-                       -> IO (OAuth2Result BSL.ByteString)  -- ^ Response as ByteString
+doSimplePostRequest :: FromJSON err => Manager                 -- ^ HTTP connection manager.
+                       -> OAuth2                               -- ^ OAuth options
+                       -> URI                                  -- ^ URL
+                       -> PostBody                             -- ^ Request body.
+                       -> IO (OAuth2Result err BSL.ByteString) -- ^ Response as ByteString
 doSimplePostRequest manager oa url body = fmap handleResponse go
                                   where go = do
                                              req <- uriToRequest url
@@ -95,49 +97,49 @@
 -- * AUTH requests
 --------------------------------------------------
 
--- | Conduct GET request and return response as JSON.
-authGetJSON :: FromJSON a
-                 => Manager                      -- ^ HTTP connection manager.
+-- | Conduct an authorized GET request and return response as JSON.
+authGetJSON :: FromJSON err => FromJSON a
+                 => Manager                 -- ^ HTTP connection manager.
                  -> AccessToken
-                 -> URI                          -- ^ Full URL
-                 -> IO (OAuth2Result a)          -- ^ Response as JSON
+                 -> URI
+                 -> IO (OAuth2Result err a) -- ^ Response as JSON
 authGetJSON manager t uri = parseResponseJSON <$> authGetBS manager t uri
 
--- | Conduct GET request.
-authGetBS :: Manager                              -- ^ HTTP connection manager.
+-- | Conduct an authorized GET request.
+authGetBS :: FromJSON err => Manager                 -- ^ HTTP connection manager.
              -> AccessToken
-             -> URI                               -- ^ URL
-             -> IO (OAuth2Result BSL.ByteString)  -- ^ Response as ByteString
+             -> URI
+             -> IO (OAuth2Result err BSL.ByteString) -- ^ Response as ByteString
 authGetBS manager token url = do
   req <- uriToRequest url
   authRequest req upReq manager
   where upReq = updateRequestHeaders (Just token) . setMethod HT.GET
 
 -- | same to 'authGetBS' but set access token to query parameter rather than header
-authGetBS' :: Manager                             -- ^ HTTP connection manager.
+authGetBS' :: FromJSON err => Manager                -- ^ HTTP connection manager.
              -> AccessToken
-             -> URI                               -- ^ URL
-             -> IO (OAuth2Result BSL.ByteString)  -- ^ Response as ByteString
+             -> URI
+             -> IO (OAuth2Result err BSL.ByteString) -- ^ Response as ByteString
 authGetBS' manager token url = do
   req <- uriToRequest (url `appendAccessToken` token)
   authRequest req upReq manager
   where upReq = updateRequestHeaders Nothing . setMethod HT.GET
 
 -- | Conduct POST request and return response as JSON.
-authPostJSON :: FromJSON a
-                 => Manager                      -- ^ HTTP connection manager.
+authPostJSON :: FromJSON err => FromJSON a
+                 => Manager                 -- ^ HTTP connection manager.
                  -> AccessToken
-                 -> URI                          -- ^ Full URL
+                 -> URI
                  -> PostBody
-                 -> IO (OAuth2Result a)          -- ^ Response as JSON
+                 -> IO (OAuth2Result err a) -- ^ Response as JSON
 authPostJSON manager t uri pb = parseResponseJSON <$> authPostBS manager t uri pb
 
 -- | Conduct POST request.
-authPostBS :: Manager                             -- ^ HTTP connection manager.
+authPostBS :: FromJSON err => Manager                -- ^ HTTP connection manager.
              -> AccessToken
-             -> URI                               -- ^ URL
+             -> URI
              -> PostBody
-             -> IO (OAuth2Result BSL.ByteString)  -- ^ Response as ByteString
+             -> IO (OAuth2Result err BSL.ByteString) -- ^ Response as ByteString
 authPostBS manager token url pb = do
   req <- uriToRequest url
   authRequest req upReq manager
@@ -146,11 +148,11 @@
         upReq = upHeaders . upBody
 
 -- | Conduct POST request with access token in the request body rather header
-authPostBS' :: Manager                            -- ^ HTTP connection manager.
+authPostBS' :: FromJSON err => Manager               -- ^ HTTP connection manager.
              -> AccessToken
-             -> URI                               -- ^ URL
+             -> URI
              -> PostBody
-             -> IO (OAuth2Result BSL.ByteString)  -- ^ Response as ByteString
+             -> IO (OAuth2Result err BSL.ByteString) -- ^ Response as ByteString
 authPostBS' manager token url pb = do
   req <- uriToRequest url
   authRequest req upReq manager
@@ -161,10 +163,10 @@
 -- |Send an HTTP request including the Authorization header with the specified
 --  access token.
 --
-authRequest :: Request                          -- ^ Request to perform
+authRequest :: FromJSON err => Request          -- ^ Request to perform
                -> (Request -> Request)          -- ^ Modify request before sending
                -> Manager                       -- ^ HTTP connection manager.
-               -> IO (OAuth2Result BSL.ByteString)
+               -> IO (OAuth2Result err BSL.ByteString)
 authRequest req upReq manager = fmap handleResponse (httpLbs (upReq req) manager)
 
 --------------------------------------------------
@@ -172,32 +174,25 @@
 --------------------------------------------------
 
 -- | Parses a @Response@ to to @OAuth2Result@
-handleResponse :: Response BSL.ByteString -> OAuth2Result BSL.ByteString
+handleResponse :: FromJSON err => Response BSL.ByteString -> OAuth2Result err BSL.ByteString
 handleResponse rsp =
     if HT.statusIsSuccessful (responseStatus rsp)
         then Right $ responseBody rsp
-        else Left $ BSL.append "hoauth2.HttpClient.handleResponse/Gaining token failed: " (responseBody rsp)
+        else Left $ parseOAuth2Error (responseBody rsp)
 
 -- | Parses a @OAuth2Result BSL.ByteString@ into @FromJSON a => a@
-parseResponseJSON :: FromJSON a
-              => OAuth2Result BSL.ByteString
-              -> OAuth2Result a
+parseResponseJSON :: FromJSON err => FromJSON a
+              => OAuth2Result err BSL.ByteString
+              -> OAuth2Result err a
 parseResponseJSON (Left b) = Left b
-parseResponseJSON (Right b) = case eitherDecode b of
-                            Left msg -> Left ("hoauth2.HttpClient.parseResponseJSON/Could not decode JSON: "
-                                              `BSL.append`
-                                              b
-                                               `BSL.append`
-                                             ", with error: "
-                                               `BSL.append`
-                                               BSL.pack msg
-                                             )
-                            Right x -> Right x
+parseResponseJSON (Right b) = case decode b of
+                            Nothing -> Left (parseOAuth2Error b)
+                            Just x -> Right x
 
 -- | Parses a @OAuth2Result BSL.ByteString@ that contains not JSON but a Query String
-parseResponseString :: FromJSON a
-              => OAuth2Result BSL.ByteString
-              -> OAuth2Result a
+parseResponseString :: FromJSON err => FromJSON a
+              => OAuth2Result err BSL.ByteString
+              -> OAuth2Result err a
 parseResponseString (Left b) = Left b
 parseResponseString (Right b) = case parseQuery $ BSL.toStrict b of
                               [] -> Left errorMessage
@@ -207,12 +202,12 @@
   where
     queryToValue = Object . HM.fromList . map paramToPair
     paramToPair (k, mv) = (T.decodeUtf8 k, maybe Null (String . T.decodeUtf8) mv)
-    errorMessage = "hoauth2.HttpClient.parseResponseString/Could not decode URL: " `BSL.append` b
+    errorMessage = parseOAuth2Error b
 
 -- | Try 'parseResponseJSON' and 'parseResponseString'
-parseResponseFlexible :: FromJSON a
-                         => OAuth2Result BSL.ByteString
-                         -> OAuth2Result a
+parseResponseFlexible :: FromJSON err => FromJSON a
+                         => OAuth2Result err BSL.ByteString
+                         -> OAuth2Result err a
 parseResponseFlexible r = case parseResponseJSON r of
                            Left _ -> parseResponseString r
                            x      -> x
diff --git a/src/Network/OAuth/OAuth2/Internal.hs b/src/Network/OAuth/OAuth2/Internal.hs
--- a/src/Network/OAuth/OAuth2/Internal.hs
+++ b/src/Network/OAuth/OAuth2/Internal.hs
@@ -10,21 +10,25 @@
 
 module Network.OAuth.OAuth2.Internal where
 
+import           Prelude              hiding (error)
 import           Control.Arrow        (second)
+import           Control.Applicative
 import           Control.Monad.Catch
 import           Data.Aeson
 import           Data.Aeson.Types
 import qualified Data.ByteString      as BS
 import qualified Data.ByteString.Lazy as BSL
 import           Data.Maybe
-import           Data.Text            (Text)
+import           Data.Monoid
+import           Data.Text            (Text, pack)
 import           Data.Text.Encoding
 import           GHC.Generics
+import           URI.ByteString
+import           URI.ByteString.Aeson ()
 import           Lens.Micro
 import           Lens.Micro.Extras
 import           Network.HTTP.Conduit as C
 import qualified Network.HTTP.Types   as H
-import           URI.ByteString
 
 --------------------------------------------------
 -- * Data Types
@@ -63,12 +67,36 @@
 instance ToJSON OAuth2Token where
     toEncoding = genericToEncoding defaultOptions { fieldLabelModifier = camelTo2 '_' }
 
+data OAuth2Error a =
+  OAuth2Error
+    { error :: Either Text a
+    , errorDescription :: Maybe Text
+    , errorUri :: Maybe (URIRef Absolute) }
+  deriving (Show, Eq, Generic)
+
+instance FromJSON err => FromJSON (OAuth2Error err) where
+  parseJSON (Object a) =
+    do
+      err <- (a .: "error") >>= (\str -> Right <$> parseJSON str <|> Left <$> parseJSON str)
+      desc <- a .:? "error_description"
+      uri <- a .:? "error_uri"
+      return $ OAuth2Error err desc uri
+  parseJSON _ = fail "Expected an object"
+
+instance ToJSON err => ToJSON (OAuth2Error err) where
+  toEncoding = genericToEncoding defaultOptions { constructorTagModifier = camelTo2 '_', allNullaryToStringTag = True }
+
+parseOAuth2Error :: FromJSON err => BSL.ByteString -> OAuth2Error err
+parseOAuth2Error string =
+  either (\err -> OAuth2Error (Left "Decode error") (Just $ pack $ "Error: " <> err <> "\n Original Response:\n" <> show (decodeUtf8 $ BSL.toStrict string)) Nothing) id (eitherDecode string)
+
+
 --------------------------------------------------
 -- * Types Synonym
 --------------------------------------------------
 
 -- | Is either 'Left' containing an error or 'Right' containg a result
-type OAuth2Result a = Either BSL.ByteString a
+type OAuth2Result err a = Either (OAuth2Error err) a
 
 -- | type synonym of post body content
 type PostBody = [(BS.ByteString, BS.ByteString)]
diff --git a/src/Network/OAuth/OAuth2/TokenRequest.hs b/src/Network/OAuth/OAuth2/TokenRequest.hs
new file mode 100644
--- /dev/null
+++ b/src/Network/OAuth/OAuth2/TokenRequest.hs
@@ -0,0 +1,22 @@
+{-# LANGUAGE DeriveGeneric              #-}
+
+module Network.OAuth.OAuth2.TokenRequest where
+
+import           Data.Aeson
+import           Data.Aeson.Types
+import           GHC.Generics
+
+instance FromJSON Errors where
+  parseJSON = genericParseJSON defaultOptions { constructorTagModifier = camelTo2 '_', allNullaryToStringTag = True }
+instance ToJSON Errors where
+  toEncoding = genericToEncoding defaultOptions { constructorTagModifier = camelTo2 '_', allNullaryToStringTag = True }
+
+-- | Token Error Responses https://tools.ietf.org/html/rfc6749#section-5.2
+data Errors =
+    InvalidRequest
+  | InvalidClient
+  | InvalidGrant
+  | UnauthorizedClient
+  | UnsupportedGrantType
+  | InvalidScope
+  deriving (Show, Eq, Generic)