The influence of the concentration of Cph(3)(+)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 (C5Me4SiMe2NBul)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 x 10(9) g PP (mol Zr)(-1) [C3H6](-1) h(-1) (25 degreesC, 1 bar), other, typically less active zirconocenes do not show a trityl response at all. Possible reasons for this effect are discussed. (C) 2004 Elsevier B.V. All rights reserved.