diff --git a/CHANGELOG b/CHANGELOG
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,3 +1,7 @@
+2015-08-07 Will Sewell <will@pusher.com>
+
+    * Add ObjectUInt constructor for messagepack unsigned ints
+
 2015-05-07 Mike Pye <mike@pusher.com>
 
     * Remove coercion of msgpack String type to haskell Text
diff --git a/Data/MessagePack.hs b/Data/MessagePack.hs
--- a/Data/MessagePack.hs
+++ b/Data/MessagePack.hs
@@ -21,10 +21,14 @@
 import Data.Int
 import Data.MessagePack.Spec
 import Data.Serialize
+import Data.Word
 import qualified Data.ByteString  as BS
 import qualified Data.Map as M
 
 data Object = ObjectNil
+            -- | Unsigned integers from the MsgPack protocol: uint 8, uint 16, uint 32, uint 64
+            | ObjectUInt   Word64
+            -- | Signed integers and fixnums from the MsgPack protocol: positive fixnum, negative fixnum, int 8, int 16, int 32, int 64
             | ObjectInt    Int64
             | ObjectBool   Bool
             | ObjectFloat  Float
@@ -37,19 +41,20 @@
     deriving (Eq, Ord, Show)
 
 instance Serialize Object where
-
-    put (ObjectInt i)
-          | i >= 0   && i <= 127        = putWord8 $ fromIntegral i
-          | i >= -32 && i <= -1         = putWord8 $ fromIntegral i
+    put (ObjectUInt i)
           | i >= 0   && i < 0x100       = putWord8 uint8  >> putWord8    (fromIntegral i)
           | i >= 0   && i < 0x10000     = putWord8 uint16 >> putWord16be (fromIntegral i)
           | i >= 0   && i < 0x100000000 = putWord8 uint32 >> putWord32be (fromIntegral i)
-          | i >= 0                      = putWord8 uint64 >> putWord64be (fromIntegral i)
-          | i >= -0x80                  = putWord8 int8   >> putWord8    (fromIntegral i)
-          | i >= -0x8000                = putWord8 int16  >> putWord16be (fromIntegral i)
-          | i >= -0x80000000            = putWord8 int32  >> putWord32be (fromIntegral i)
-          | otherwise                   = putWord8 int64  >> putWord64be (fromIntegral i)
+          | otherwise                   = putWord8 uint64 >> putWord64be (fromIntegral i)
 
+    put (ObjectInt i)
+          | i >= 0           && i <= 127        = putWord8 $ fromIntegral i
+          | i >= -32         && i <= -1         = putWord8 $ fromIntegral i
+          | i >= -0x80       && i < 0x80        = putWord8 int8   >> putWord8    (fromIntegral i)
+          | i >= -0x8000     && i < 0x8000      = putWord8 int16  >> putWord16be (fromIntegral i)
+          | i >= -0x80000000 && i < 0x80000000  = putWord8 int32  >> putWord32be (fromIntegral i)
+          | otherwise                           = putWord8 int64  >> putWord64be (fromIntegral i)
+
     put ObjectNil          = putWord8 nil
 
     put (ObjectBool b)     = putWord8 $ if b then true else false
@@ -128,10 +133,10 @@
 
           | k .&. posFixintMask == posFixint  = return $ ObjectInt $ fromIntegral k
           | k .&. negFixintMask == negFixint  = return $ ObjectInt $ fromIntegral (fromIntegral k :: Int8)
-          | k == uint8                        = ObjectInt <$> fromIntegral <$> getWord8
-          | k == uint16                       = ObjectInt <$> fromIntegral <$> getWord16be
-          | k == uint32                       = ObjectInt <$> fromIntegral <$> getWord32be
-          | k == uint64                       = ObjectInt <$> fromIntegral <$> getWord64be
+          | k == uint8                        = ObjectUInt <$> fromIntegral <$> getWord8
+          | k == uint16                       = ObjectUInt <$> fromIntegral <$> getWord16be
+          | k == uint32                       = ObjectUInt <$> fromIntegral <$> getWord32be
+          | k == uint64                       = ObjectUInt <$> getWord64be
           | k == int8                         = ObjectInt <$> fromIntegral <$> (get :: Get Int8)
           | k == int16                        = ObjectInt <$> fromIntegral <$> (get :: Get Int16)
           | k == int32                        = ObjectInt <$> fromIntegral <$> (get :: Get Int32)
diff --git a/messagepack.cabal b/messagepack.cabal
--- a/messagepack.cabal
+++ b/messagepack.cabal
@@ -1,5 +1,5 @@
 name               : messagepack
-version            : 0.4.0
+version            : 0.5.0
 synopsis           : Serialize instance for Message Pack Object
 description        : Serialize instance for Message Pack Object
 homepage           : https://github.com/rodrigosetti/messagepack
diff --git a/tests/Main.hs b/tests/Main.hs
--- a/tests/Main.hs
+++ b/tests/Main.hs
@@ -15,6 +15,7 @@
 
 instance Arbitrary Object where
     arbitrary = sized $ \n -> oneof [ return ObjectNil
+                                    , ObjectUInt   <$> arbitrary
                                     , ObjectInt    <$> arbitrary
                                     , ObjectBool   <$> arbitrary
                                     , ObjectFloat  <$> arbitrary