diff --git a/compose/compose-file/index.md b/compose/compose-file/index.md
index 680e16d558..c8b914a388 100644
--- a/compose/compose-file/index.md
+++ b/compose/compose-file/index.md
@@ -911,9 +911,10 @@ the service's task containers.
 - `mode`: The permissions for the file that will be mounted in `/run/secrets/`
   in the service's task containers, in octal notation. For instance, `0444`
   represents world-readable. The default in Docker 1.13.1 is `0000`, but will
-  be `0444` in the future. Secrets cannot be writable, but the executable bit
-  can be set. If you aren't familiar with UNIX file permission modes, you
-  may find this
+  be `0444` in the future. Secrets cannot be writable because they are mounted
+  in a temporary filesystem, so if you set the writable bit, it is ignored. The
+  executable bit can be set. If you aren't familiar with UNIX file permission
+  modes, you may find this
   [permissions calculator](http://permissions-calculator.org/){: target="_blank" class="_" }
   useful.