A theoretical study Of Cp2ZrCl2/MAO system is presented based on Density Functional Theory (DFT) calculations performed with gaussian basis-sets. The MAO cocatalyst was modeled as a small finite section of the most probably species in solution. The geometry of the cation-counterion forming the ionic-pair active site was fully optimized and its electronic structure was examined in terms of the Natural Bond Orbital (NBO) population analysis. In this way the ionic-pair interaction can be related to the electronic charge changes around the zirconium cation. The results indicate the formation of a stable ionic-pair and the redistribution of charge in the cation complex and counterion. The latter was concluded by mapping the charge density difference. The Laplacian of charge density confirm the ionic interaction of the metallocene/MAO system. (C) 2002 Elsevier Science B.V. All rights reserved.