diff --git a/CHANGELOG b/CHANGELOG
index da9a811..d0bea47 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,5 +1,7 @@
 *SVN*
 
+* Exclude Opera from using the sourceIndex approach in Element#descendantOf. [Tobie Langel, Andrew Dupont]
+
 * Ensure Element#hasClassName always returns a boolean. Closes #10075. [ronnylovtangen, Tobie Langel]
 
 * Ensure selectors of the form "[href]" (attribute token with no preceding tag name) work properly. Closes #8870. [chao, kangax, Andrew Dupont]
diff --git a/src/dom.js b/src/dom.js
index f2367f4..b480015 100644
--- a/src/dom.js
+++ b/src/dom.js
@@ -362,7 +362,7 @@ Element.Methods = {
     if (element.compareDocumentPosition)
       return (element.compareDocumentPosition(ancestor) & 8) === 8;
       
-    if (element.sourceIndex) {
+    if (element.sourceIndex && !Prototype.Browser.Opera) {
       var e = element.sourceIndex, a = ancestor.sourceIndex,
        nextAncestor = ancestor.nextSibling;
       if (!nextAncestor) {