Silica Grace 948 activated at 723 K was modified with MAO in aluminum contents comprised between 0.5 and 20.0 wt.% Al/SiO2. Supported metallocene catalysts were prepared by grafting (nBuCp)(2)ZrCl2 toluene solutions directly onto silica and onto MAO-modified silicas. The catalysts were characterized by a series of complementary techniques, namely, Rutherford backscattering spectrometry (RBS), Diffuse-reflectance Fourier transform spectroscopy (DRIFTS), ultraviolet-visible spectroscopy (UV-Vis) and electron probe microanalysis (EPMA). MAO-mediated systems presented higher Zr content comparing to directly silica supported system. For the MAO-modified silica systems, Zr content increases up to 2.0-4.0 wt.% Al/SiO2 reaching 1.5 wt.% Zr/SiO2. As the aluminum content increases, metal loading decreases achieving 0.6 wt.% Zr/SiO2 for the supported catalyst having 20.0 wt.% Al/SiO2. All the systems were shown to be active in ethylene polymerization having MAO as cocatalyst. Higher activity were achieved with MAO-mediated systems having 2.0-4.0 wt.% Al/SiO2. The catalyst systems produced polyethylenes with high molecular weight and narrow molecular weight distribution. The preparative protocol was stepwisely monitored by DRIFTS. For lower MAO content (2.0 wt.% Al/SiO2) residual isolated silanol groups were thoroughly consumed or perturbed after (nBuCp)(2)ZrCl2 impregnation. (C) 2003 Elsevier B.V. All rights reserved.