diff --git a/ChangeLog b/ChangeLog
index 23dfb8c6..72160452 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,7 @@
 18/10/13	started 7.36.3
 - fix compiler warnings in ubuntu 13.10
+- reverse similarity rotation direction to match the convention used 
+  elsewhere in vips
 
 10/10/13	started 7.36.2
 - better jpeg startup
diff --git a/libvips/resample/similarity.c b/libvips/resample/similarity.c
index bffc2ba4..ffdb8ff2 100644
--- a/libvips/resample/similarity.c
+++ b/libvips/resample/similarity.c
@@ -3,6 +3,9 @@
  *
  * 3/10/13
  * 	- from similarity.c
+ * 25/10/13
+ * 	- oops, reverse rotation direction to match the convention used in the
+ * 	  rest of vips
  */
 
 /*
@@ -79,7 +82,7 @@ vips_similarity_build( VipsObject *object )
 		return( -1 );
 
 	a = similarity->scale * cos( VIPS_RAD( similarity->angle ) ); 
-	b = sin( VIPS_RAD( similarity->angle ) );
+	b = -sin( VIPS_RAD( similarity->angle ) );
 	c = -b;
 	d = a;