diff --git a/docs/compatibility.txt b/docs/compatibility.txt
index 9ee891e20..5580e6856 100644
--- a/docs/compatibility.txt
+++ b/docs/compatibility.txt
@@ -34,3 +34,9 @@ PHP Driver Compatibility
 
 To use {+odm-long+} v5.2 or later, you must install v1.21 of the
 {+php-library+} and {+php-extension+}.
+
+.. important:: {+php-extension+} v2.0 Compatibility
+
+   If you upgrade the {+php-extension+} to v2.0, you must also upgrade
+   {+odm-long+} to v5.2.1. {+odm-long+} v5.2.1 still supports v1.x
+   versions of the extension.