In a photoinduced reaction, an intermediate produced by the reaction of a reductant R with photogenerated holes can promote a reduction reaction that is not observed in the absence of R. We illustrate this scheme by considering the reduction of Ni-II at TiO2. In this case Ni-II, which is not deposited on illuminated TiO2, can be reduced by the intermediate CO2.- produced from the anodic oxidation of oxalate ion (C2O42-) at irradiated TiO2 in water. Irradiation of an aqueous Ni-II solution containing 10 mu g Ni/mL in the presence of TiO2 particles led to 85% removal of the Ni from the solution. The mechanism of the reaction was probed by measurement of current-doubling effects for oxalate solutions at a single-crystal TiO2 electrode.