Liberation of atomic hydrogen is detected upon photon (h upsilon = 10 eV) irradiation of thermally grown SiO2 on Si followed by neutralization of the created positive charge by electrons. This indicates the formation and trapping of protons in the oxide. Additional evidence for this is provided by an isotopic effect in the positive charge decay observed in samples enriched with H-1 and D-2. The most efficient proton generation process is suggested as hole trapping by O-3 = SiH centers, leading to the formation of O-3 = Si . defects (E' centers) in the oxide.