A modified extended Huckel molecular orbital (EHMO) method, which includes core-core repulsion terms, was used to study the silane adsorption on SiO2 surfaces. The SiO2 treated at 300 degrees C was chemically modified with an alcoxysilane, phenyltriethoxysilane (PTES). Therefore, we study the possible adsorption of PTES on two planes (with isolated and geminal silanols) and the possible species formed once the reaction with a SiO2 surface is finished. Starting from this analysis, it is possible to evaluate the PTES selectivity to the different anchoring points on the SiO2 surface.