Index: test/unit/core.js
===================================================================
--- test/unit/core.js	(revision 6271)
+++ test/unit/core.js	(working copy)
@@ -366,6 +366,7 @@
 	ok( pass, "Execute a function, Relative" );
 });
 
+
 test("index(Object)", function() {
 	expect(10);
 
@@ -386,6 +387,18 @@
 	equals( elements.index( elements.eq(1) ), 1, "Pass in a jQuery object" );
 });
 
+test("index(String|undefined)", function() {
+	expect(4);
+
+	equals( jQuery('#text2').index(), 2, "Check for index amongst siblings" );
+	equals( jQuery('#form').children().eq(4).index(), 4, "Check for index amongst siblings" );
+	equals( jQuery('#radio2').index('#form :radio') , 1, "Check for index within a selector" );
+    equals( jQuery('#radio2').index('#form :text') , -1, "Check for index not found within a selector" );
+	
+});
+
+
+
 test("attr(String)", function() {
 	expect(27);
 	equals( jQuery('#text1').attr('value'), "Test", 'Check for value attribute' );
Index: src/core.js
===================================================================
--- src/core.js	(revision 6271)
+++ src/core.js	(working copy)
@@ -158,11 +158,19 @@
 	// Determine the position of an element within
 	// the matched set of elements
 	index: function( elem ) {
+			
+	  // legacy implementation
+	  if ( typeof elem === 'object'){
 		// Locate the position of the desired element
-		return jQuery.inArray(
-			// If it receives a jQuery object, the first element is used
-			elem && elem.jquery ? elem[0] : elem
-		, this );
+	    return jQuery.inArray(
+		        // If it receives a jQuery object, the first element is used
+	              elem && elem.jquery ? elem[0] : elem
+	             , this );
+	  }
+	  //return the index of the element in a new jQuery obj from selector or by default, amongst its own siblings.
+	  return jQuery.inArray(this[0],
+	            elem ? jQuery(elem) : jQuery.sibling(this[0].parentNode.firstChild,undefined) );
+	 
 	},
 
 	attr: function( name, value, type ) {