The present invention relates to supported catalyst composition for an addition polymerization, a process to prepare the same and the use thereof in an addition polymerization, wherein the composition comprises: (A) a particulated, solid support material; (B) an activator which is an ammonium salt of a compatible, noncoordinating anion, Anull, said anion, Anull containing a remnant formed by reaction of a non-ionic Lewis acid and a moiety containing an active hydrogen; and (C) a Group 4 metal complex catalyst.