diff --git a/packages/material-ui/src/ExpansionPanelSummary/ExpansionPanelSummary.js b/packages/material-ui/src/ExpansionPanelSummary/ExpansionPanelSummary.js
index 43bfaee4293c63..bd0d1522218f08 100644
--- a/packages/material-ui/src/ExpansionPanelSummary/ExpansionPanelSummary.js
+++ b/packages/material-ui/src/ExpansionPanelSummary/ExpansionPanelSummary.js
@@ -31,7 +31,8 @@ export const styles = theme => {
         opacity: 0.38,
       },
     },
-    /* Styles applied to the root element if `expanded={true}`. */
+    // eslint-disable-next-line max-len
+    /* Styles applied to the root element, children wrapper element and `IconButton` component if `expanded={true}`. */
     expanded: {},
     /* Styles applied to the root and children wrapper elements when focused. */
     focused: {},
diff --git a/pages/api/expansion-panel-summary.md b/pages/api/expansion-panel-summary.md
index 5f562ad7cc0a8a..70636b633c4d2a 100644
--- a/pages/api/expansion-panel-summary.md
+++ b/pages/api/expansion-panel-summary.md
@@ -34,7 +34,7 @@ This property accepts the following keys:
 | Name | Description |
 |:-----|:------------|
 | <span class="prop-name">root</span> | Styles applied to the root element.
-| <span class="prop-name">expanded</span> | Styles applied to the root element if `expanded={true}`.
+| <span class="prop-name">expanded</span> | Styles applied to the root element, children wrapper element and `IconButton` component if `expanded={true}`.
 | <span class="prop-name">focused</span> | Styles applied to the root and children wrapper elements when focused.
 | <span class="prop-name">disabled</span> | Styles applied to the root element if `disabled={true}`.
 | <span class="prop-name">content</span> | Styles applied to the children wrapper element.