--- a/activesupport/lib/active_support/buffered_logger.rb
+++ b/activesupport/lib/active_support/buffered_logger.rb
@@ -100,7 +100,12 @@ module ActiveSupport
     def flush
       @guard.synchronize do
         unless buffer.empty?
-          old_buffer = buffer
+          if defined? Encoding
+            old_buffer = buffer.map { |b| (b.encoding == Encoding::ASCII_8BIT)? b.force_encoding(Encoding::UTF_8).encode : b }
+          else
+            old_buffer = buffer
+          end
+          
           @log.write(old_buffer.join)
         end