diff --git a/activesupport/CHANGELOG b/activesupport/CHANGELOG
index e3c0c8a71b..9ea01ada2e 100644
--- a/activesupport/CHANGELOG
+++ b/activesupport/CHANGELOG
@@ -1,5 +1,7 @@
 *SVN*
 
+* Ensure Chars#tidy_bytes only tidies broken bytes. Closes #6397 [Manfred Stienstra]
+
 * Add 'unloadable', a method used to mark any constant as requiring an unload after each request. [Nicholas Seckar]
 
 * Make core_ext/string/access.rb multibyte safe. Closes #6388 [Manfred Stienstra]
diff --git a/activesupport/lib/active_support/multibyte/handlers/utf8_handler.rb b/activesupport/lib/active_support/multibyte/handlers/utf8_handler.rb
index 6c8eb88702..5b64734297 100644
--- a/activesupport/lib/active_support/multibyte/handlers/utf8_handler.rb
+++ b/activesupport/lib/active_support/multibyte/handlers/utf8_handler.rb
@@ -259,13 +259,18 @@ module ActiveSupport::Multibyte::Handlers
         g_unpack(str).length
       end
       
-      # Strips all the non-utf-8 bytes from the string resulting in a valid utf-8 string
+      # Replaces all the non-utf-8 bytes by their iso-8859-1 or cp1252 equivalent resulting in a valid utf-8 string
       def tidy_bytes(str)
-        str.unpack('C*').map { |n|
-          n < 128 ? n.chr :
-          n < 160 ? [UCD.cp1252[n] || n].pack('U') :
-          n < 192 ? "\xC2" + n.chr : "\xC3" + (n-64).chr
-        }.join
+        str.split(//u).map do |c|
+          if !UTF8_PAT.match(c)
+            n = c.unpack('C')[0]
+            n < 128 ? n.chr :
+            n < 160 ? [UCD.cp1252[n] || n].pack('U') :
+            n < 192 ? "\xC2" + n.chr : "\xC3" + (n-64).chr
+          else
+            c
+          end
+        end.join
       end
       
       protected
diff --git a/activesupport/test/multibyte_handler_test.rb b/activesupport/test/multibyte_handler_test.rb
index 06bc904c04..95d6faec7c 100644
--- a/activesupport/test/multibyte_handler_test.rb
+++ b/activesupport/test/multibyte_handler_test.rb
@@ -228,7 +228,10 @@ module UTF8HandlingTest
   def test_tidy_bytes
     result = [0xb8, 0x17e, 0x8, 0x2c6, 0xa5].pack('U*')
     assert_equal result, @handler.tidy_bytes(@bytestring)
-    assert_equal "a#{result}a", @handler.tidy_bytes('a' + @bytestring + 'a')
+    assert_equal "a#{result}a", @handler.tidy_bytes('a' + @bytestring + 'a'),
+      'tidy_bytes should leave surrounding characters intact'
+    assert_equal "é#{result}é", @handler.tidy_bytes('é' + @bytestring + 'é'),
+      'tidy_bytes should leave surrounding characters intact'
     assert_nothing_raised { @handler.tidy_bytes(@bytestring).unpack('U*') }
     
     assert_equal "\xC3\xA7", @handler.tidy_bytes("\xE7") # iso_8859_1: small c cedilla
@@ -236,7 +239,7 @@ module UTF8HandlingTest
     assert_equal "\xE2\x80\x9C", @handler.tidy_bytes("\x93") # win_1252: left smart quote
     assert_equal "\xE2\x82\xAC", @handler.tidy_bytes("\x80") # win_1252: euro
     assert_equal "\x00", @handler.tidy_bytes("\x00") # null char
-    assert_equal [0xef, 0xbf, 0xbd].pack('U*'), @handler.tidy_bytes("\xef\xbf\xbd") # invalid char
+    assert_equal [0xfffd].pack('U'), @handler.tidy_bytes("\xef\xbf\xbd") # invalid char
   end
   
   protected