This paper describes the indirect photocatalytic reduction of four metal ions: Cd2+, Zn2+, Mn2+, and Tl+ using formate radical anions (CO2.-) generated in UV-irradiated aqueous TiO2 suspensions. Trends in the reactivity of these four metal ions are compared with corresponding rate constants reported in the literature for the homogeneous reactions of these metal ions with CO2.- in aqueous media. In both cases, the reaction rates follow the same order: Cd2+ > Tl+ > Mn2+ > Zn2+. Using Tl+ as a model metal ion, a simple kinetics scheme is developed for the indirect reduction route, and the predictions from this model are shown to be in excellent agreement with experimental data.