The present invention relates to a supported, treated catalyst system and its use in a process for polymerizing olefin(s). More particularly, it provides a supported, treated catalyst system produced by a process comprising the steps of: (a) forming a supported bimetallic catalyst system comprising a first catalyst component and a metallocene catalyst compound; and (b) contacting the supported bimetallic catalyst system of (a) with at least one methylalumoxane-activatable compound.