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Radiation experiments have established that H/sup +/ released in SiO/sub 2/ migrates to the Si-SiO/sub 2/ interface where it can induce new defects. For oxides exposed first to high-temperature annealing and then to molecular hydrogen, mobile positive charge believed to be H/sup +/ can be cycled to and from the interface by reversing the oxide electric field. We report first-principles calculations that identify atomic-scale mechanisms for the two types of behavior and the conditions that are necessary for each. Si-Si bonds on the oxide side, i.e.,"suboxide bonds," can trap H+ in deep wells with an asymmetric barrier (1.5 eV on the Si side, 1 eV on the SiO/sub 2/ side). In radiation experiments these centers can act as fixed positive charge. In the mobile-positive-charge experiments, the protons can be cycled between opposing Si-SiO/sub 2/ interfaces if the density of suboxide bonds is high.