Theoretical calculations have been carried out on the grafting of two chains organo-silane compounds on SiO2 hydroxylated solid surfaces. Considering two different silylated coupling agents, two grafting stable complexes are obtained. These complexes are stabilized by two interactions: (i) the chain is grafted to the cluster with a covalent bond Si-O-Si; (ii) the chain interacts with the cluster via an hydrogen bond H-O...O in the other side of the chain. The electronic, geometrical and vibrational properties of these systems are analysed. These results give new insight about the grafting of long chains organo-silane on silica surfaces. (C) 2004 Elsevier B.V. All rights reserved.