diff --git a/CHANGELOG b/CHANGELOG
index 31a86cb..326ba74 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,3 +1,5 @@
+* Avoid object creation and an unnecessary function call in `Class#addMethods`, when working around JScript DontEnum bug. Replace with feature test and a simple boolean check at runtime. (kangax)
+
 * Optimize Element#immediateDescendants. (kangax, Tobie Langel)
 
 * Remove unnecessary function object creation and `Number#times` in `Element._getContentFromAnonymousElement`. (kangax)
diff --git a/src/lang/class.js b/src/lang/class.js
index 3f59bb6..b277e2b 100644
--- a/src/lang/class.js
+++ b/src/lang/class.js
@@ -9,6 +9,17 @@
  *  inheritance](http://prototypejs.org/learn/class-inheritance).
 **/
 var Class = (function() {
+  
+  // Some versions of JScript fail to enumerate over properties, names of which 
+  // correspond to non-enumerable properties in the prototype chain
+  var IS_DONTENUM_BUGGY = (function(){
+    for (var p in { toString: 1 }) {
+      // check actual property name, so that it works with augmented Object.prototype
+      if (p === 'toString') return false;
+    }
+    return true;
+  })();
+  
   /**
    *  Class.create([superclass][, methods...]) -> Class
    *    - superclass (Class): The optional superclass to inherit methods from.
@@ -138,9 +149,10 @@ var Class = (function() {
     var ancestor   = this.superclass && this.superclass.prototype,
         properties = Object.keys(source);
 
-    // IE6 doesn't enumerate toString and valueOf properties,
-    // Force copy if they're not coming from Object.prototype.
-    if (!Object.keys({ toString: true }).length) {
+    // IE6 doesn't enumerate `toString` and `valueOf` (among other built-in `Object.prototype`) properties,
+    // Force copy if they're not Object.prototype ones.
+    // Do not copy other Object.prototype.* for performance reasons
+    if (IS_DONTENUM_BUGGY) {
       if (source.toString != Object.prototype.toString)
         properties.push("toString");
       if (source.valueOf != Object.prototype.valueOf)