diff --git a/ChangeLog b/ChangeLog
index 7715ed97bf..20172e0c11 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,10 @@
+Thu Jun 10 09:10:08 2010  Yukihiro Matsumoto  <matz@ruby-lang.org>
+
+	* io.c (rb_io_putc): documentation updated to mention putc would
+	  not work well with multi-byte characters.  [ruby-core:30697]
+
+	* io.c (rb_f_putc): ditto.
+
 Wed Jun  9 22:51:50 2010  Tanaka Akira  <akr@fsij.org>
 
 	* time.c (find_time_t): always outerpolate from past.
diff --git a/io.c b/io.c
index d010f38374..e293728847 100644
--- a/io.c
+++ b/io.c
@@ -6012,9 +6012,11 @@ rb_f_print(int argc, VALUE *argv)
  *  call-seq:
  *     ios.putc(obj)    -> obj
  *
- *  If <i>obj</i> is <code>Numeric</code>, write the character whose
- *  code is <i>obj</i>, otherwise write the first character of the
- *  string representation of  <i>obj</i> to <em>ios</em>.
+ *  If <i>obj</i> is <code>Numeric</code>, write the character whose code is
+ *  the least-significant byte of <i>obj</i>, otherwise write the first byte
+ *  of the string representation of <i>obj</i> to <em>ios</em>. Note: This
+ *  method is not safe for use with multi-byte characters as it will truncate
+ *  them.
  *
  *     $stdout.putc "A"
  *     $stdout.putc 65
@@ -6040,6 +6042,9 @@ rb_io_putc(VALUE io, VALUE ch)
  *  Equivalent to:
  *
  *    $stdout.putc(int)
+ * 
+ * Refer to the documentation for IO#putc for important information regarding
+ * multi-byte characters.
  */
 
 static VALUE