diff --git a/ChangeLog b/ChangeLog
index e886a6508e..24cfad0fc3 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,10 @@
+Sat Oct 25 20:09:09 2014  Martin Duerst <duerst@it.aoyama.ac.jp>
+
+	* lib/unicode_normalize/normalize.rb: added US_ASCII
+	  as trivially supported encoding (is always normalized,
+	  and may appear mixed in with UTF-8 or other Unicode
+	  encodings).
+
 Sat Oct 25 20:01:01 2014  Martin Duerst <duerst@it.aoyama.ac.jp>
 
 	* test/test-unicode_normalize.rb: added
diff --git a/lib/unicode_normalize/normalize.rb b/lib/unicode_normalize/normalize.rb
index 97aaeddbee..0c99024c86 100644
--- a/lib/unicode_normalize/normalize.rb
+++ b/lib/unicode_normalize/normalize.rb
@@ -138,6 +138,8 @@ module UnicodeNormalize
       else
         raise ArgumentError, "Invalid normalization form #{form}."
       end
+    elsif encoding == Encoding::US_ASCII
+      string
     elsif  UNICODE_ENCODINGS.include? encoding
       normalize(string.encode(Encoding::UTF_8), form).encode(encoding)
     else
@@ -166,6 +168,8 @@ module UnicodeNormalize
       else
         raise ArgumentError, "Invalid normalization form #{form}."
       end
+    elsif encoding == Encoding::US_ASCII
+      true
     elsif  UNICODE_ENCODINGS.include? encoding
       normalized? string.encode(Encoding::UTF_8), form
     else