I think i am getting an exception but the code i wont to write into the event log isnt working. Its a permission thing, i've created an event source in the registry but it still isnt writing too it. Im going to try to track down the users ASP is running under on my server and then all that user write permissions. Thanks for the help, its always nice to go back to basics :)

Please ignore this now i've worked it out, although if anyone has a better way let me know. I've used the context to pass the control name through. string sref = Page.ClientScript.GetCallbackEventReference("context", "arg", "display","context","display_Error", true); string Callback = "function GetData(arg, context){" + sref + ";}"; Page.ClientScript.RegisterClientScriptBlock(this.GetType(), "CallBack", Callback, true); this means that in my javascript i've got calls like GetData(Value,"Lookup1"); Cheers Phil