Water molecules and water-related species are present to a varying degree in SiO2 and other dielectrics and at Si-dielectric interfaces. Their presence, even in small concentrations, constitutes a critical reliability problem for present day ultrathin dielectrics. Here we present first-principles density functional calculations that probe the reactivity of water molecules at the Si-dielectric interface. We report results on different possible reaction pathways for water at the interface, including dissociation of water that can lead to the release of H+ ions. The released protons can migrate along the interface and depassivate dangling bonds. Results are also presented for the reaction of water with a hydrogen atom migrating laterally along the interface that lead to the creation of hydronium species. These atomic-scale mechanisms may account for at least some of the creation of interface traps and oxide trapped charge, the two features that give rise to negative bias temperature instability (NBTI), a well known reliability phenomenon that occurs in MOSFETs under normal operating and stress conditions.