The reactivity Of CP2ZrCl2 towards partially dehydroxylated silica was evaluated and the effects of chemical modification of this silica were studied. Different modified silicas were prepared by reaction of the original partially dehydroxylated silica with silicon ethers, EtOSiMe3 and (Me3Si)(2)O, or a silazane, (Me3Si)(2)NH. The resulting materials were activated with MAO and the catalytic systems were evaluated in ethylene polymerization. The different reactions were monitored by FI'-IR spectroscopy. The catalysts were characterized by elemental analysis as well as by infrared and UV-vis spectroscopy. Grafting of organosilanes, occurs by reaction with reactive siloxane bridges. The new SiR3 groups formed on the surface react with CP2ZrCl2 to form volatile ClSiMe3 and oxo zirconium species. These latter species are active, after the addition of MAO, in ethylene polymerization. The effects caused by changing the nature of the modifier in the grafting reaction with the metallocene, as well as the catalytic activities of the resulting materials, are presented and discussed. (C) 2004 Elsevier B.V. All rights reserved.