The interaction of H-2 With the defect sites of the SiO2 surface has been studied by means of gradient-corrected density functional theory calculations on cluster models. The mechanism of hydrogen dissociation, the energy of reactants and products, and the corresponding activation energies and transition states have been determined for the following defect sites: Si singly occupied sp(3) dangling bonds (E' centers), =Si-.; nonbridging oxygen centers (NBO), =Si-O-.; divalent Si, =Si:; and neutral oxygen vacancies, =Si-Si=. H-2 cracking on the NBO sites is exothermic by similar to 0.4 eV and has an energy barrier of similar to 0.1 eV (or less considering nonadiabatic effects) which suggest the occurrence of the process even at low temperature. On Si dangling bonds the formation of =Si-H and neutral H atom is endothermic and occurs with an activation energy of less than 0.5 eV; the reaction can occur at room temperature. The interaction of molecular hydrogen with the diamagnetic oxygen deficient centers, =Si: and =Si-Si=, leads to the formation of stable =Si-H groups with exothermic processes and relatively high activation energies of about 2 eV. Thus, H-2 cracking is predicted to occur at room temperature on paramagnetic defects and only at high temperatures on the diamagnetic centers.