A modified extended Huckel method (EHMO), which includes core-core repulsion terms, was used to study a propylene polymerization heterogeneous catalyst by modeling the adsorption of the ansa-metallocene ethylen-bisindenylzirconium dichloride (C 2 H 5 (C 9 H 6 ) 2 ZrCl 2 from now on EtInd 2 ZrCl 2 ) on SiO 2 surfaces. The adsorption was considered to occur at two faces of SiO 2 (with geminal and isolated OH's) and at two different temperatures for SiO 2 pretreatment (200 and 600°C). Taking into account that the interaction with Al compounds is considered as a way to improve the catalyst performance, specially with methylaluminoxane (MAO), a local model of aluminoxane (AlO) was added to Si-O bond with the subsequent metallocene adsorption. The results show that AlO improves the zirconocene adsorption on SiO 2 modeled at 200°C while it inhibits that process at 600°C.