A series of heterogeneous catalyst systems was prepared by the immobilization of bis( n-butylcyclopentadienyl) zirconium dichloride, (nBuCp)(2)ZrCl2, on silica supports activated at different temperatures. Silica dehydroxylation was evaluated in terms of retained metal loading and activity in ethylene homopolymerization. Characterization of the catalyst systems was accomplished by Rutherford back-scattering spectrometry and infrared spectroscopy. The highest metal loading (0.48 wt.% Zr/SiO2) was achieved with silica treated under vacuum at room temperature (298 K), but the catalyst showed only a minor polymerization activity which may be attributed to a large number of inactive Zr-support bidentate species formed at a high surface density of OH in silica. IR spectroscopic data show that, regardless of support activation temperature, a significant number of isolated OH groups remains after zirconocene fixation. The presence of bulky ligands in the catalyst molecule seems to prevent the remaining OH groups from reacting with additional metallocene complexes, keeping the metal loading around 0.35 wt.% Zr/SiO2 for silica activated between 373 and 723 K. High polymerization activity observed for the system based on 373 K-activated silica suggests a role for these OH groups in the generation of active alkylated species when methylaluminoxane and trimethylaluminum (contained in MAO itself) are added at the beginning of the polymerization reaction. IR analysis shows that TMA, which is a less sterically demanding compound than MAO, can effectively access the remaining OH groups consuming them thoroughly. Practically all the prepared systems presented activity in ethylene polymerization with MAO as cocatalyst, the highest activity (5.1 x 10(5) PE g mol(-1) Zr h(-1)) having been obtained with silica dehydroxylated at 723 K. (C) 1999 Elsevier Science B.V. All rights reserved.