A series of supported metallocene catalysts were prepared by immobilizing (nBuCp)(2)ZrCl2 and Cp2NbCl2 on bare and on MAO-modified silica. Both metallocenes were sequentially grafted on silica under different combinations. Metal loading (Zr, Nb and Al) was determined by X-ray fluorescence spectroscopy (XRF). Final metal content was shown to be dependent on passivation of silica with MAO, and on the grafting sequence. Higher metal contents (0.42 wt.% Nb/SiO2 and 1 wt.% Zr/SiO2) were achieved when the metallocene was grafted onto MAO-modified silica. Cp2NbCl2 derivatives were not active in polymerizing ethylene using MAO as co-catalyst (Al/Nb = 2000). All the systems containing zirconocene derivatives showed high catalyst activity. Nevertheless, higher activities were achieved immobilizing (nBuCp)(2)ZrCl2 on silica that was previously modified with MAO and Cp2NbCl2. Some immobilization protocols led to catalyst systems which were even more active than the homogenous counterpart. Effects of support structure and chemical modification on the catalyst performance are presented and discussed. (C) 2002 Elsevier Science B.V. All rights reserved.