The immobilization of Cp2ZrCl2 Was performed by entrapment within the binary oxide SiO2-WO3 using a non-hydrolytic sol-gel route. The catalyst and oxide matrix were characterized by complementary techniques (Rutherford backscattering spectrometry, ultraviolet-visible diffuse reflectance and infrared transmission spectroscopy, differential pulse voltammetry, adsorption-desorption of N-2, and X-ray diffraction). The catalyst performance in terms of catalytic activity and polymer properties was evaluated by ethylene polymerization. Catalyst characterization suggested that the entrapped complex exhibited lower Zr electronic density than the corresponding unsupported metallocene. The polymerization results also revealed that this low Zr electronic density is optimized in terms of complex activation with low MAO concentration (low [Al/Zr] ratio) and reactivity. This fine tuning results in a catalyst system that is active with a low [Al/Zr] ratio and that achieves high catalytic activity. The results demonstrate that the entrapping method allows the generation of a catalyst system in which part of the activation process is attributed to the support. in this sense, the support may partially play MAO functions by stabilizing the active catalytic species. (C) 2009 Elsevier B.V. All rights reserved.