The present disclosure provides methods for preparing a catalyst system comprising contacting in an aliphatic solvent at least one support material, at least one hydrocarbyl aluminum compound and at least one non-hydrolytic active oxygen-containing compound to form a supported alumoxane (catalyst system precursor) and contacting the supported alumoxane with at least one catalyst compound having a Group 3 through Group 12 metal atom or lanthanide metal atom. The supported alumoxane may be heated prior to contact with the catalyst compound.