Three kinds of zirconocene catalysts, supported on polysiloxanes substituted with methyl-indenyl, methyl-fluorenyl and two indenyl groups, respectively, were prepared and applied to propene and ethylene polymerizations using methylaluminoxane as cocatalyst. Both the activity and stereospecificity of these catalysts were found to lie between those of corresponding homogeneous and SiO2-supported catalysts. Surprisingly, all the catalysts gave a mixture of atactic and isotactic poly(propylene), plausible reasons for which are briefly discussed.