diff --git a/ChangeLog b/ChangeLog
index 5fdf2d789c..2b5f6b918b 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Tue Dec  2 15:30:30 2014  Martin Duerst  <duerst@it.aoyama.ac.jp>
+
+	* array.c (rb_ary_plus): in documentation, added note about
+	  inefficiency of repeated += operations.
+
 Tue Dec  2 07:20:21 2014  Eric Wong  <e@80x24.org>
 
 	* iseq.c (iseq_data_to_ary): keep hidden variables
diff --git a/array.c b/array.c
index 18cde10782..2c390f9749 100644
--- a/array.c
+++ b/array.c
@@ -3525,9 +3525,17 @@ rb_ary_fill(int argc, VALUE *argv, VALUE ary)
  *     c                         #=> [ "a", "b", "c", "d", "e", "f" ]
  *     a                         #=> [ "a", "b", "c" ]
  *
+ *  Note that
+ *     x += y
+ *  is the same as
+ *     x = x + y
+ *  This means that it produces a new array. As a consequence,
+ *  repeated use of += on arrays can be quite inefficient.
+ *
  *  See also Array#concat.
  */
 
+
 VALUE
 rb_ary_plus(VALUE x, VALUE y)
 {