SiO2-water clusters are studied using first-principles Born-Oppenheimer molecular dynamics based on density functional theory and generalized gradient approximations. Systematic investigations of structure and energetics as functions of cluster size demonstrate the roles of water molecules in chemical reactions. The water-assisted formation of a Si(OH)(4) molecule from a single SiO2 molecule is revealed at the atomic level. The dynamics of dissociation of water molecules and formation of Si-OH bonds is investigated via simulations at finite temperature. A complex process that involves double and triple hydrogen atom transfer is discovered to be the reaction path.