Photocatalytic/photoelectrochemical water splitting using metal oxide semiconductors is a promising technology for direct and simple solar-energy conversion. The addition of carbonate salts to an aqueous reaction solution has been known to promote stoichiometric O₂ evolution and H₂O₂ production via H₂O oxidation. To elucidate the effect of carbonates, density functional theory calculations are performed to study the photoinduced H₂O and H₂CO₃ oxidation mechanisms on TiO₂ and BiVO₄. The oxidation reactions proceeded via peroxide intermediates, such as H₂O₂ for H₂O, H₂C₂O₆ for H₂CO₃, and H₂CO₄ for the coexistence of H₂O and H₂CO₃ molecules. Regardless of the reactant and metal oxide, the free energy changes in the four proton-coupled electron-transfer (PCET) steps of the oxidation mechanism indicate that the first PCET requires the highest energy input and is the rate-limiting step. All PCET steps of the H₂O oxidation, except the second one, are more endergonic than those of the H₂CO₃ oxidation. The H₂O reactant requires a larger energy barrier at the highest energy profile, as well as at the final state, than the H₂CO₃ reactant. The computational results verify that the adsorbed H₂CO₃ molecule is easily photo-oxidized compared with the adsorbed H₂O molecule, facilitating the formation of the peroxide intermediate and improving O₂ evolution and H₂O₂ production.