Photoactive metal oxides, such as hydrated TiO2, are known to reduce carbon dioxide to methane, but the mechanism for this photoreaction is insufficiently understood. In particular, it is not known whether the reduction of crucial reaction intermediates, including the formate anion, involves one- or two-electron reactions. In this study, we demonstrate that formic acid and its derivatives can be reduced to the formyl radical via a concerted reaction in which the electron transfer is coupled to oxygen transfer to a Ti3+ center on the oxide surface. Several other examples of such heteroatom-transfer reactions are demonstrated, suggesting a general pattern. The implications of these reactions for photocatalytic methanogenesis, perchlorate diagenesis, and planetary chemistry on Mars are discussed.