A supported polymerisation catalyst system comprises: (a) a polymerisation catalyst, (b) a cocatalyst, and (c) a porous support, and is characterised in that the porous support has been pretreated with (i) a chemical dehydration agent and (ii) a hydroxy compound wherein the hydroxy compound is not a cocatalyst or component thereof. The preferred polymerisation catalyst is a transition metal compound for example a metallocene and by use of the supported catalyst systems improved activity may be achieved.