Novel, highly effective catalyst compositions are described in which a low cost co-catalyst can be employed at very low aluminum loadings. Such compounds are composed of a cation derived from d-block or f-block metal compound, such as a metallocene, by loss of a leaving group, and an aluminoxate anion derived by transfer of a proton from a stable or metastable hydroxyaluminoxane to such leaving group. These catalyst compositions have extremely high catalytic activity and typically have high solubility in paraffinic solvents. Moreover they yield reduced levels of ash and result in improved clarity in polymers formed from such catalysts. Surprisingly, when isolated and stored, and optionally purified, under anhydrous inert conditions and surroundings, the catalyst compounds are more stable than if kept in solution. Thus these catalyst compounds can be stored, shipped, and used under inert anhydrous conditions as preformed catalysts thus simplifying the polymerization operations. Substantial benefits are also realized by forming, and by using in the formation of activated catalysts, a hydroxyaluminoxane supported on a catalyst support or carrier.