Various metallocene complexes of the IV group transition metals supported on HY zeolites were investigated in the polymerization of ethylene and propylene. A comparative study with the-same catalysts operating in solution was also performed. Generally, a decrease in the catalytic activity after support was, observed, whose extent depends on the preliminary treatment of the support with aluminum alkyl. In general, better stability with the time and higher molecular weight of the polymers were obtained, The high isotactic stereospecificity of-chiral metallocene catalysts for propylene polymerization was maintained after support. In the case of the aspecific bis(indenyl) complex, a modest but significant effect on the stereospecific control was observed due to the steric influence of the support.