diff --git a/Doc/library/subprocess.rst b/Doc/library/subprocess.rst
index 45eff7ffcc3502..10e40635e24869 100644
--- a/Doc/library/subprocess.rst
+++ b/Doc/library/subprocess.rst
@@ -195,7 +195,10 @@ compatibility with older versions, see the :ref:`call-function-trio` section.
     .. attribute:: output
 
         Output of the child process if it was captured by :func:`run` or
-        :func:`check_output`.  Otherwise, ``None``.
+        :func:`check_output`.  Otherwise, ``None``.  This is always
+        :class:`bytes` when any output was captured regardless of the
+        ``text=True`` setting.  It may remain ``None`` instead of ``b''``
+        when no output was observed.
 
     .. attribute:: stdout
 
@@ -204,7 +207,9 @@ compatibility with older versions, see the :ref:`call-function-trio` section.
     .. attribute:: stderr
 
         Stderr output of the child process if it was captured by :func:`run`.
-        Otherwise, ``None``.
+        Otherwise, ``None``.  This is always :class:`bytes` when stderr output
+        was captured regardless of the ``text=True`` setting.  It may remain
+        ``None`` instead of ``b''`` when no stderr output was observed.
 
     .. versionadded:: 3.3