A catalyst system suitable for the polymerization of olefins comprising an olefin polymerization catalyst and an activator comprising the reaction product of (i) an aluminoxane or (ii) the reaction product of an alkylaluminum and water with a silicon-containing compound, for example diphenylsilane diol. Preferred catalysts are metallocene complexes. The activators provide an alternative to the traditional use of aluminoxanes alone.