A supported polymerisation catalyst system comprises a porous support, a polymerisation catalyst and a cocatalyst wherein the support has been treated with an inert material such as polyisobutene. The support is preferably silica and is preferably treated with the inert material prior to contact with the other components. Preferably the polymerisation catalyst is a metallocene and the polymerisation catalyst system reduces the initial peak isotherm allowing for a smoother activity increase in particular for polymerisations carried out in the gas phase.