A supported polymerization catalyst system is prepared by a method comprising the following steps: (i) addition of a cocatalyst to a porous support, (ii) mixing a polymerisation catalyst with a polymerisable monomer, and (iii) contacting together the components resulting from steps (i) and (ii). The porous support is preferably silica and the polymerisation catalyst is preferably a metallocene. The polymerisable monomer is typically 1-hexene and the supported catalyst system provides advantages a slowly decaying activity profile particularly when operating in the gas phase.