diff --git a/src/pkg/io/io.go b/src/pkg/io/io.go
index c0bc54d0d4e..d3707eb1dba 100644
--- a/src/pkg/io/io.go
+++ b/src/pkg/io/io.go
@@ -136,6 +136,10 @@ type WriterTo interface {
 // At the end of the input stream, ReadAt returns 0, os.EOF.
 // ReadAt may return a non-zero number of bytes with a non-nil err.
 // In particular, a ReadAt that exhausts the input may return n > 0, os.EOF.
+//
+// If ReadAt is reading from an data stream with a seek offset,
+// ReadAt should not affect nor be affected by the underlying
+// seek offset.
 type ReaderAt interface {
 	ReadAt(p []byte, off int64) (n int, err os.Error)
 }