diff --git a/CHANGELOG b/CHANGELOG
index 282e3f2..0476ff7 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,5 +1,7 @@
 *SVN*
 
+* Prevent DOM node expandos _countedByPrototype and _prototypeEventID from being serialized into (inner|outer)HTML in IE. Closes #10909. [dcpedit, Tobie Langel, Andrew Dupont]
+
 * Make Ajax.Request#isSameOrigin JavaScript Lint compliant. [Andrew Dupont]
 
 * Properly serialize form fields with names that collide with built-in JS properties (like "length" or "toString"). Closes #9609. [gryn, kangax]
diff --git a/src/event.js b/src/event.js
index 46688da..589a718 100644
--- a/src/event.js
+++ b/src/event.js
@@ -128,11 +128,11 @@ Event.extend = (function() {
 
 Object.extend(Event, (function() {
   var cache = Event.cache;
-  
+
   function getEventID(element) {
-    if (element._eventID) return element._eventID;
+    if (element._prototypeEventID) return element._prototypeEventID[0];
     arguments.callee.id = arguments.callee.id || 1;
-    return element._eventID = ++arguments.callee.id;
+    return element._prototypeEventID = [++arguments.callee.id];
   }
   
   function getDOMEventName(eventName) {
diff --git a/src/selector.js b/src/selector.js
index de5d590..e679356 100644
--- a/src/selector.js
+++ b/src/selector.js
@@ -312,14 +312,15 @@ Object.extend(Selector, {
     
     // marks an array of nodes for counting
     mark: function(nodes) {
+      var _true = Prototype.emptyFunction;
       for (var i = 0, node; node = nodes[i]; i++)
-        node._counted = true;
+        node._countedByPrototype = _true;
       return nodes;
     },
     
     unmark: function(nodes) {
       for (var i = 0, node; node = nodes[i]; i++)
-        node._counted = undefined;    
+        node._countedByPrototype = undefined;    
       return nodes;  
     },
 
@@ -327,15 +328,15 @@ Object.extend(Selector, {
     // "ofType" flag indicates whether we're indexing for nth-of-type
     // rather than nth-child
     index: function(parentNode, reverse, ofType) {
-      parentNode._counted = true;
+      parentNode._countedByPrototype = Prototype.emptyFunction;
       if (reverse) {
         for (var nodes = parentNode.childNodes, i = nodes.length - 1, j = 1; i >= 0; i--) {
           var node = nodes[i];
-          if (node.nodeType == 1 && (!ofType || node._counted)) node.nodeIndex = j++;
+          if (node.nodeType == 1 && (!ofType || node._countedByPrototype)) node.nodeIndex = j++;
         }
       } else {
         for (var i = 0, j = 1, nodes = parentNode.childNodes; node = nodes[i]; i++)
-          if (node.nodeType == 1 && (!ofType || node._counted)) node.nodeIndex = j++;
+          if (node.nodeType == 1 && (!ofType || node._countedByPrototype)) node.nodeIndex = j++;
       }
     },
     
@@ -344,8 +345,8 @@ Object.extend(Selector, {
       if (nodes.length == 0) return nodes;
       var results = [], n;
       for (var i = 0, l = nodes.length; i < l; i++)
-        if (!(n = nodes[i])._counted) {
-          n._counted = true;
+        if (!(n = nodes[i])._countedByPrototype) {
+          n._countedByPrototype = Prototype.emptyFunction;
           results.push(Element.extend(n));
         }
       return Selector.handlers.unmark(results);
@@ -547,7 +548,7 @@ Object.extend(Selector, {
       var h = Selector.handlers, results = [], indexed = [], m;
       h.mark(nodes);
       for (var i = 0, node; node = nodes[i]; i++) {
-        if (!node.parentNode._counted) {
+        if (!node.parentNode._countedByPrototype) {
           h.index(node.parentNode, reverse, ofType);
           indexed.push(node.parentNode);
         }
@@ -585,7 +586,7 @@ Object.extend(Selector, {
       var exclusions = new Selector(selector).findElements(root);
       h.mark(exclusions);
       for (var i = 0, results = [], node; node = nodes[i]; i++)
-        if (!node._counted) results.push(node);
+        if (!node._countedByPrototype) results.push(node);
       h.unmark(exclusions);
       return results;
     },
@@ -618,12 +619,20 @@ Object.extend(Selector, {
     '~=': function(nv, v) { return (' ' + nv + ' ').include(' ' + v + ' '); },
     '|=': function(nv, v) { return ('-' + nv.toUpperCase() + '-').include('-' + v.toUpperCase() + '-'); }
   },
+  
+  split: function(expression) {
+    var expressions = [];
+    expression.scan(/(([\w#:.~>+()\s-]+|\*|\[.*?\])+)\s*(,|$)/, function(m) {
+      expressions.push(m[1].strip());
+    });
+    return expressions;
+  },
 
   matchElements: function(elements, expression) {
     var matches = $$(expression), h = Selector.handlers;
     h.mark(matches);
     for (var i = 0, results = [], element; element = elements[i]; i++)
-      if (element._counted) results.push(element);
+      if (element._countedByPrototype) results.push(element);
     h.unmark(matches);
     return results;
   },
@@ -636,11 +645,7 @@ Object.extend(Selector, {
   },
   
   findChildElements: function(element, expressions) {
-    var exprs = expressions.join(',');
-    expressions = [];
-    exprs.scan(/(([\w#:.~>+()\s-]+|\*|\[.*?\])+)\s*(,|$)/, function(m) {
-      expressions.push(m[1].strip());
-    });
+    expressions = Selector.split(expressions.join(','));
     var results = [], h = Selector.handlers;    
     for (var i = 0, l = expressions.length, selector; i < l; i++) {
       selector = new Selector(expressions[i].strip());
@@ -651,13 +656,22 @@ Object.extend(Selector, {
 });
 
 if (Prototype.Browser.IE) {
-  // IE returns comment nodes on getElementsByTagName("*").
-  // Filter them out.
-  Selector.handlers.concat = function(a, b) {
-    for (var i = 0, node; node = b[i]; i++)
-      if (node.tagName !== "!") a.push(node);      
-    return a;
-  };
+  Object.extend(Selector.handlers, {
+    // IE returns comment nodes on getElementsByTagName("*").
+    // Filter them out.
+    concat: function(a, b) {
+      for (var i = 0, node; node = b[i]; i++)
+        if (node.tagName !== "!") a.push(node);
+      return a;
+    },
+    
+    // IE improperly serializes _countedByPrototype in (inner|outer)HTML.
+    unmark: function(nodes) {
+      for (var i = 0, node; node = nodes[i]; i++)
+        node.removeAttribute('_countedByPrototype');
+      return nodes;  
+    }
+  });  
 }
 
 function $$() {
diff --git a/test/unit/event.html b/test/unit/event.html
index 18372fa..7d4f15a 100644
--- a/test/unit/event.html
+++ b/test/unit/event.html
@@ -26,6 +26,7 @@
 <div id="outer" style="display: none">
   <p id="inner">One two three <span id="span">four</span></p>
 </div>
+<div id="container"><div></div></div>
 
 <!-- Tests follow -->
 <script type="text/javascript" language="javascript" charset="utf-8">
@@ -174,7 +175,7 @@
       var span = $("span"), observer = function() { }, eventID;
       
       span.observe("test:somethingHappened", observer);
-      eventID = span._eventID;
+      eventID = span._prototypeEventID;
       
       assert(Event.cache[eventID]);
       assert(Object.isArray(Event.cache[eventID]["test:somethingHappened"]));
@@ -240,6 +241,12 @@
       assertElementMatches(event.findElement('p'), 'p#inner');
       assertEqual(null, event.findElement('div.does_not_exist'));
       assertElementMatches(event.findElement('.does_not_exist, span'), 'span#span');
+    }},
+    
+    testEventIDDuplication: function() { with(this) {
+      $('container').down().observe("test:somethingHappened", Prototype.emptyFunction);
+      $('container').innerHTML += $('container').innerHTML;
+      assertUndefined($('container').down(1)._prototypeEventID);
     }}
   });
 
diff --git a/test/unit/selector.html b/test/unit/selector.html
index 1d57c1c..bf85d81 100644
--- a/test/unit/selector.html
+++ b/test/unit/selector.html
@@ -88,7 +88,7 @@
     <input type="hidden" id="commaChild" name="foo" value="#commaOne,#commaTwo" />
     <input type="hidden" id="commaTwo" name="foo2" value="oops" />
   </form>
-  
+  <div id="counted_container"><div class="is_counted"></div></div>
 </div> <!-- #fixtures -->
 
 <!-- Log output -->
@@ -442,8 +442,29 @@
       assert(typeof results[0].show == 'function');
       assert(typeof results[1].show == 'function');
       assert(typeof results[2].show == 'function');
+    }},
+    
+    testCountedIsNotAnAttribute: function() { with(this) {
+      var el = $('list');
+      Selector.handlers.mark([el]);
+      assert(!el.innerHTML.include("_counted"));
+      Selector.handlers.unmark([el]);
+      assert(!el.innerHTML.include("_counted"));      
+    }},
+
+    testCopiedNodesGetIncluded: function(){ with(this) {
+      assertElementsMatch(
+        Selector.matchElements($('counted_container').descendants(), 'div'),
+        'div.is_counted'
+      );
+      $('counted_container').innerHTML += $('counted_container').innerHTML;
+      assertElementsMatch(
+        Selector.matchElements($('counted_container').descendants(), 'div'), 'div.is_counted', 
+        'div.is_counted'
+      );
     }}
   });
+  
 // ]]>
 </script>
 </body>