Use this in default.aspx<script language="javascript" type="text/javascript"> ESRI.ADF.System.maximumLapseTime = Infinity; ESRI.ADF.System.showLapseAlert = function() { window.location = 'SessionEnd.aspx'; }</script>The main point is to use ESRI.ADF.System.maximumLapseTime instead of maximumLapseTime