diff --git a/CHANGELOG b/CHANGELOG
index 643b69b..fd7e73c 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,5 +1,7 @@
 *SVN*
 
+* Performance optimizations for Element#descendantOf. Costliness should no longer be dependent on the difference in depth between the parent and the child. [Andrew Dupont]
+
 * Apply the workaround for the Firefox "blinking element" opacity=1 bug only to Firefox 1.5.  [Thomas Fuchs]
 
 * Add event.stopped, a boolean that is set to `true` when Event#stop is called. [Andrew Dupont, sam]
diff --git a/src/dom.js b/src/dom.js
index a3d24e2..17161eb 100644
--- a/src/dom.js
+++ b/src/dom.js
@@ -358,6 +358,20 @@ Element.Methods = {
   
   descendantOf: function(element, ancestor) {
     element = $(element), ancestor = $(ancestor);
+
+    if (element.compareDocumentPosition)
+      return (element.compareDocumentPosition(ancestor) & 24) === 8;
+      
+    if (element.sourceIndex) {
+      var e = element.sourceIndex, a = ancestor.sourceIndex,
+       nextAncestor = ancestor.nextSibling;
+      if (!nextAncestor) {
+        do { ancestor = ancestor.parentNode; }
+        while (!(nextAncestor = ancestor.nextSibling) && ancestor.parentNode);
+      }
+      if (nextAncestor) return (e > a && e < nextAncestor.sourceIndex);      
+    }
+    
     while (element = element.parentNode)
       if (element == ancestor) return true;
     return false;
diff --git a/test/unit/dom.html b/test/unit/dom.html
index fb2438b..1ed5565 100644
--- a/test/unit/dom.html
+++ b/test/unit/dom.html
@@ -319,7 +319,13 @@
     
 </p>
 <p id="test-full">content</p>
-<div id="ancestor"><div id="child"><div><div id="great-grand-child"></div></div></div></div>
+<div id="ancestor">
+  <div id="child">
+    <div id="grand-child">
+      <div id="great-grand-child"></div>
+    </div></div><!-- intentional formatting; don't change this line -->
+  <div id="sibling"><div id="grand-sibling"></div></div>
+</div>
 <div id="not-in-the-family"></div>
 
 <div id="insertions-container"><div id="insertions-main"><p>some content.</p></div></div>
@@ -1003,8 +1009,28 @@
     testDescendantOf: function() {with(this) {
       assert($('child').descendantOf('ancestor'));
       assert($('child').descendantOf($('ancestor')));
-      assert($('great-grand-child').descendantOf('ancestor'));
-      assert(!$('great-grand-child').descendantOf('not-in-the-family'));
+      
+      assert(!$('ancestor').descendantOf($('child')));
+
+      assert($('great-grand-child').descendantOf('ancestor'), 'great-grand-child < ancestor');
+      assert($('grand-child').descendantOf('ancestor'), 'grand-child < ancestor');
+      assert($('great-grand-child').descendantOf('grand-child'), 'great-grand-child < grand-child');
+      assert($('grand-child').descendantOf('child'), 'grand-child < child');
+      assert($('great-grand-child').descendantOf('child'), 'great-grand-child < child');
+      
+      window.debug = true;
+      assert($('sibling').descendantOf('ancestor'), 'sibling < ancestor');
+      assert($('grand-sibling').descendantOf('sibling'), 'grand-sibling < sibling');
+      assert($('grand-sibling').descendantOf('ancestor'), 'grand-sibling < ancestor');
+      
+      assert($('grand-sibling').descendantOf(document.body), 'grand-sibling < body');      
+      window.debug = false;
+      
+      assert(!$('great-grand-child').descendantOf('great-grand-child'), 'great-grand-child < great-grand-child');
+      assert(!$('great-grand-child').descendantOf('sibling'), 'great-grand-child < sibling');
+      assert(!$('sibling').descendantOf('child'), 'sibling < child');
+      assert(!$('great-grand-child').descendantOf('not-in-the-family'), 'great-grand-child < not-in-the-family');
+      assert(!$('child').descendantOf('not-in-the-family'), 'child < not-in-the-family');
     }},  
     
     testChildOf: function() {with(this) {