Comparative theoretical analysis of optimized geometries and electronic structures of Cp2ZrCl2/methylaluminoxane (MAO) system and its interactions with a third component was performed. The additive studied was a Lewis acid compound: AlCl3. Density functional theory (DFT), that included exchange and correlation effects, was employed with Gaussian basis sets. Additive enhances the cocatalyst Lewis acid character and, indirectly, it modifies the electrophilicity of Zr atom. By comparing the geometrical structure and charge distribution of the site with and without AlCl3 it is possible to understand the reasons of polymerization performance improvement. Previous experimental results for these systems have showed a notable increase in ethylene and propylene polymerization activity. (C) 2003 Elsevier B.V. All rights reserved.