Ethylene polymerization using catalysts derived from activation of zirconocene aluminohydride complexes, supported on silica, pretreated with methylaluminoxane (MAO), is described. The novel catalyst compositions were compared to those using conventional zirconocene dichloride complexes and characterized by SEM/EDX and DRIFT spectroscopy. Supported catalysts were prepared which featured various surface Al:Zr ratios. When using excess MAO as both activator and scavenger, the catalysts containing the most Zr per g of support gave rise to the most active formulations: the high activities in the presence of excess MAO are due, in part, to catalyst leaching prior to and/or during polymerization. When triisobutylaluminum (TIBAL) was used as scavenger. the supported catalysts that featured a higher surface Al:Zr ratio had higher activity than those prepared at the lower AI:Zr ratios. The activity of the aluminohydride complexes was significantly higher than that of the corresponding dichloride complexes, when activated by MAO while the in the presence of TIBAL. there was little difference in performance between the two catalyst precursors. (C) 2009 Elsevier B.V. All rights reserved.