The effect of A1 content on MAO-modified silicas was evaluated on catalyst activity, on polymer properties and on residual metal content in the resulting polyethylenes. MAO-modified silicas were prepared by impregnating MAO toluene solutions in concentration range between 0.5 and 20.0 wt% Al/SiO2. Commercial MAO-modified silica (Witco) containing 24.4 wt% Al/SiO2 was used for comparative reasons. The resulting modified-silicas were employed as supports for grafting (nBuCp)(2)ZrCl2. Using external MAO as cocatalyst (Al/Zr = 2000) no difference in catalyst activity was observed. Nevertheless, for At/Zr = 500, catalyst activities were shown to be higher for supported zirconocene systems containing 0.0-2.0 wt% Al/SiO2 range. According to DSC analysis, one T-m peak was detected for polymer obtained with catalyst prepared with 0.5 wt% Al/SiO2 (135 C), but two T-m, peaks were observed for polymers obtained with catalysts prepared with 10.0 wt% Al/SiO2 (136 and 141 C) and 20.0 wt% Al/SiO2 (133 and 141 C).