The influence of the concentration of CPh3+B(C6F5)4- as the activator for metallocene polymerisation catalysts has been quantified. Whereas typically a 1:1 molar ratio of activator and metallocene dialkyl is employed, in line with the expected stoichiometry of alkyl abstraction, significant increases in the productivity of some—but not all—propene polymerisation catalysts have been found. The effect is particularly marked for catalysts with more open structures, such as the constrained-geometry complex (C5Me4SiMe2NBut)TiCl2, where the activity could be increased by about an order of magnitude. Most catalysts show an optimum CPh3+/Zr ratio, beyond which further increases of [CPh3+] has no effect. While some zirconocenes reach activities of up to 1.2×109 g PP (mol Zr)-1 [C3H6]-1 h-1 (25 °C, 1 bar), other, typically less active zirconocenes do not show a trityl response at all. Possible reasons for this effect are discussed.