We performed first-principles quantum-mechanical calculations on the mechanisms of proton formation in SiO2, Cluster models of various size were used to represent two oxygen deficient centers in silica, the E' center, a Si dangling bond, and the neutral oxygen vacancy. The dissociation of H-2, the formation of protons, and the mechanism and energetics of proton migration were investigated. We found that the E' center is an active site for hydrogen cracking and proton generation; the reaction of the E' center with H-2 should lead to a Si dangling bond near a Si-H group and a mobile proton. The proton is stabilized at bridging oxygen sites with formation of oxonium ions, (=Si-)(2)OH+, where the oxygen atom is three-coordinated. Another mechanism of proton formation is from the reaction of atomic H with a neutral O vacancy. The process leads to a free electron and a stable proton bridge structure where the H+ ion is midway between two Si atoms.