Came across a need to pass NULL to a paremeter in a Reporting Services report through the ReportService web service. Turns out (thanks Teo Lachev) that if this is what you need, omit the parameter altogether from the parameter collection that is passed into the web service. Teo mentioned that he figured this out by doing a server trace. Very clever. He remembered that Reporting Services is using the ReportService.asmx to render the reports itself and so all he had to do was render the report using the null checkbox in the parameter box and see what it was doing. Well done.