diff --git a/Data/Char8.hs b/Data/Char8.hs
--- a/Data/Char8.hs
+++ b/Data/Char8.hs
@@ -1,6 +1,11 @@
 {-# LANGUAGE MagicHash #-}
 
--- | Char8 library to be used with Data.ByteString.Char8
+-- | Char8 library to be used with Data.ByteString.Char8.
+-- All function assumes that only 8bit part of 'Char' is used
+-- and it is encoded in Latin-1 (ISO-8859-1).
+-- All utility functions are supposed to work as if
+-- those of 'Data.Char'. Exceptions are described in
+-- the function documentations.
 
 module Data.Char8 (
   -- * Character classification
@@ -99,7 +104,7 @@
 isAscii c = _nul <= c && c <= _del
 
 isLatin1 :: Char -> Bool
-isLatin1 _ = True
+isLatin1 (C# c#) = ord# c# <# 0xff#
 
 isAsciiUpper :: Char -> Bool
 isAsciiUpper c = 'A' <= c && c <= 'Z'
@@ -109,6 +114,7 @@
 
 ----------------------------------------------------------------
 
+-- | Micro sign/mu (0xb5) and small letter Y with diaeresis (0xff) remain the same.
 toUpper :: Char -> Char
 toUpper c@(C# c#)
   | c == _germandbls = c
@@ -120,6 +126,7 @@
   | isUpper c = C# (chr# (ord# c# +# 32#))
   | otherwise = c
 
+-- | Micro sign/mu (0xb5) and small letter Y with diaeresis (0xff) remain the same.
 toTitle :: Char -> Char
 toTitle = toUpper
 
diff --git a/Data/Word8.hs b/Data/Word8.hs
--- a/Data/Word8.hs
+++ b/Data/Word8.hs
@@ -1,4 +1,8 @@
--- | Word8 library to be used with Data.ByteString
+-- | Word8 library to be used with Data.ByteString.
+-- All function assumes that 'Word8' is encoded in Latin-1 (ISO-8859-1).
+-- All utility functions are supposed to work as if
+-- those of 'Data.Char'. Exceptions are described in
+-- the function documentations.
 
 module Data.Word8 (
   -- * Re-exporting
@@ -125,6 +129,7 @@
 
 ----------------------------------------------------------------
 
+-- | Micro sign/mu (0xb5) and small letter Y with diaeresis (0xff) remain the same.
 toUpper :: Word8 -> Word8
 toUpper w
   | w == _germandbls = w
@@ -136,6 +141,7 @@
   | isUpper w = w + _space
   | otherwise = w
 
+-- | Micro sign/mu (0xb5) and small letter Y with diaeresis (0xff) remain the same.
 toTitle :: Word8 -> Word8
 toTitle = toUpper
 
diff --git a/word8.cabal b/word8.cabal
--- a/word8.cabal
+++ b/word8.cabal
@@ -1,5 +1,5 @@
 Name:                   word8
-Version:                0.0.1
+Version:                0.0.2
 Author:                 Kazu Yamamoto <kazu@iij.ad.jp>
 Maintainer:             Kazu Yamamoto <kazu@iij.ad.jp>
 License:                BSD3