diff --git a/ChangeLog b/ChangeLog
index 66b32ca3e8..0a1dc9294d 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Fri Aug 19 13:14:00 2011  Kenta Murata  <mrkn@mrkn.jp>
+
+	* thread.c: add a description for the behavior of select(2) on
+	  Mac OS X 10.7 (Lion).
+
 Fri Aug 19 11:28:58 2011  Shugo Maeda  <shugo@ruby-lang.org>
 
 	* lib/net/imap.rb (msg_att): accepts extra space before ')'.
diff --git a/thread.c b/thread.c
index 56077d83c5..6970d8f62b 100644
--- a/thread.c
+++ b/thread.c
@@ -2296,6 +2296,10 @@ rb_thread_priority_set(VALUE thread, VALUE prio)
  * - HP-UX documents how to allocate fd_set dynamically.
  *   http://docs.hp.com/en/B2355-60105/select.2.html
  * - Solaris 8 has select_large_fdset
+ * - Mac OS X 10.7 (Lion)
+ *   select(2) returns EINVAL if nfds is greater than FD_SET_SIZE and
+ *   _DARWIN_UNLIMITED_SELECT (or _DARWIN_C_SOURCE) isn't defined.
+ *   http://developer.apple.com/library/mac/#releasenotes/Darwin/SymbolVariantsRelNotes/_index.html
  *
  * When fd_set is not big enough to hold big file descriptors,
  * it should be allocated dynamically.