Using \n escaped newline character and the entity for tooltip
<?xml version="1.0"?> <mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" initialize="doSomething(event)"> <mx:Script> public function doSomething(event:Event):void { b1.toolTip = "A \n B."; } </mx:Script> <mx:Button id="b1" label="Clear" width="100" /> <mx:Button id="b2" label="Submit" width="100" toolTip="A B." /> </mx:Application>