The surface structure of silica glasses has been simulated using molecular dynamics. The surface hydroxyl concentration was estimated to be 4.5/nm2, based on surface defect statistics. Hydroxyl-silica potentials were developed and used to study the hydroxylation of silica surface. It is found that the energy of chemisorption of water declines in the sequence: three coordinated silicon (Si3) and non-bridging oxygen (NBO) on separate sites, Si3 and NBO on combined sites, two- and three-membered rings. Partial hydroxylation of the most reactive sites, which leads to an OH coverage of 2.5/nm2, was studied. Structural relaxation after hydroxylation was observed.