Fixed #1854 by using wizzud's suggestion. The only real difference is the code is only called when there is more than a single selector. So there should be no speed decrease in the current working cases. Only additional functionality for cases that used to fail.

src/core.js

@@ -342,15 +342,14 @@ jQuery.fn = jQuery.prototype = {
 	},
 
 	not: function( selector ) {
-		return this.pushStack(
-			selector.constructor == String &&
-			jQuery.multiFilter( selector, this, true ) ||
+		if (selector.constructor == String)
+			// test special case where just one selector is passed in
+			if ( /^.[^:#\[\.]*$/.test(selector) )
+				return this.pushStack( jQuery.multiFilter( selector, this, true ) );
+			else
+				selector = jQuery.multiFilter( selector, this );
 
-			jQuery.grep(this, function(elem) {
-				return selector.constructor == Array || selector.jquery ?
-					jQuery.inArray( elem, selector ) < 0 :
-					elem != selector;
-			}) );
+		return this.pushStack( jQuery.removeFromArray( selector, this ) );
 	},
 
 	add: function( selector ) {
@@ -1093,6 +1092,13 @@ jQuery.extend({
 		return -1;
 	},
 
+	removeFromArray: function( remove, from ) {
+		var isArrayLike = remove.length && remove[remove.length - 1] !== undefined;
+		return jQuery.grep(from, function(elem) {
+				return isArrayLike ? jQuery.inArray( elem, remove ) < 0 : elem != from;
+			});
+	},
+
 	merge: function( first, second ) {
 		// We have to loop this way because IE & Opera overwrite the length
 		// expando of getElementsByTagName

src/selector.js

@@ -320,7 +320,11 @@ jQuery.extend({
 			// :not() is a special case that can be optimized by
 			// keeping it out of the expression list
 			if ( m[1] == ":" && m[2] == "not" )
-				r = jQuery.filter(m[3], r, true).r;
+				// optimize if only one selector found (most common case)
+				if ( /^.[^:#\[\.]*$/.test(m[3]) )
+					r = jQuery.filter(m[3], r, true).r;
+				else
+					r = jQuery.removeFromArray(jQuery.multiFilter(m[3], r), r);
 
 			// We can get a big speed boost by filtering by class here
 			else if ( m[1] == "." )

test/index.html

@@ -61,19 +61,19 @@
 			<textarea id="area1" maxlength="30">foobar</textarea>
 			
 			<select name="select1" id="select1">
-				<option id="option1a" value="">Nothing</option>
+				<option id="option1a" class="emptyopt" value="">Nothing</option>
 				<option id="option1b" value="1">1</option>
 				<option id="option1c" value="2">2</option>
 				<option id="option1d" value="3">3</option>
 			</select>
 			<select name="select2" id="select2">
-				<option id="option2a" value="">Nothing</option>
+				<option id="option2a" class="emptyopt" value="">Nothing</option>
 				<option id="option2b" value="1">1</option>
 				<option id="option2c" value="2">2</option>
 				<option id="option2d" selected="selected" value="3">3</option>
 			</select>
 			<select name="select3" id="select3" multiple="multiple">
-				<option id="option3a" value="">Nothing</option>
+				<option id="option3a" class="emptyopt" value="">Nothing</option>
 				<option id="option3b" selected="selected" value="1">1</option>
 				<option id="option3c" selected="selected" value="2">2</option>
 				<option id="option3d" value="3">3</option>

test/unit/core.js

@@ -1052,10 +1052,12 @@ test("filter()", function() {
 });
 
 test("not()", function() {
-	expect(3);
+	expect(5);
 	ok( $("#main > p#ap > a").not("#google").length == 2, "not('selector')" );
+	isSet( $("p").not(".result").get(), q("firstp", "ap", "sndp", "en", "sap", "first"), "not('.class')" );
 	isSet( $("p").not("#ap, #sndp, .result").get(), q("firstp", "en", "sap", "first"), "not('selector, selector')" );
 	isSet( $("p").not($("#ap, #sndp, .result")).get(), q("firstp", "en", "sap", "first"), "not(jQuery)" );
+	isSet( $("#form option").not("option.emptyopt:contains('Nothing'),[selected],[value='1']").get(), q("option1c", "option1d", "option2c", "option3d" ), "not('complex selector')");
 });
 
 test("andSelf()", function() {

test/unit/selector.js

@@ -182,7 +182,7 @@ test("attributes", function() {
 });
 
 test("pseudo (:) selectors", function() {
-	expect(32);
+	expect(35);
 	t( "First Child", "p:first-child", ["firstp","sndp"] );
 	t( "Last Child", "p:last-child", ["sap"] );
 	t( "Only Child", "a:only-child", ["simon1","anchor1","yahoo","anchor2"] );
@@ -195,6 +195,9 @@ test("pseudo (:) selectors", function() {
 	t( "Text Contains", "a:contains('Google Groups')", ["groups"] );
 	t( "Element Preceded By", "p ~ div", ["foo","fx-queue","fx-tests", "moretests"] );
 	t( "Not", "a.blog:not(.link)", ["mark"] );
+	t( "Not - multiple", "#form option:not(:contains('Nothing'),#option1b,:selected)", ["option1c", "option1d", "option2b", "option2c", "option3d"] );
+	t( "Not - complex", "#form option:not([id^='opt']:gt(0):nth-child(-n+3))", [ "option1a", "option1d", "option2d", "option3d"] );
+	t( "Not - recursive", "#form option:not(:not(:selected))[id^='option3']", [ "option3b", "option3c"] );
 	
 	t( "nth Element", "p:nth(1)", ["ap"] );
 	t( "First Element", "p:first", ["firstp"] );