DFT calculations point out that the photo-oxidation of water on GaN is energetically considerably facilitated by adsorbed carbonate. As the redox potential of the couple CO₃˙⁻/CO₃²⁻ is considerably lower than that of the couple OH˙/OH⁻ but still enables the oxidation of water it is suggested that carbonate should be considered as a catalyst/co-catalyst in a variety of catalytic/photo-catalytic/electro-catalytic oxidation processes.