Current environmental issues, such as global warming and the exhaustion of fossil fuels, call for innovative technologies. In this context, efficient photocatalysts that enable the selective reduction of CO2 to CO in aqueous media are highly sought after. Although the beneficial use of Ag cocatalysts in ZnTa2O6-based photocatalytic systems has been previously studied, the CO evolution selectivity has remained moderate owing to the competing formation of H2. Herein, we report that the modification of 3.0 wt % Ag-loaded ZnTa2O6 with 10 mol % Zn leads to a significant increase in the CO evolution selectivity to 90.0% while maintaining the rate of CO formation at 25.7 μmol h–1. Thus, the modified Zn species play an important role in the suppression of the H2 evolution. Although an Ag-less, Zn-modified photocatalyst performed relatively well, the combination of a Ag cocatalyst and Zn modifier is required to ensure CO evolution at a high selectivity and rate. Furthermore, full characterization of the Zn-modified photocatalysts allowed the proposal of two putative structures. In addition to serving as ZnTa2O6 surface modifiers, the Zn species may be incorporated in the Zn/ZnTa2O6 bulk. Finally, we propose that three types of active sites exist for (1) the reduction of CO2 to CO, (2) the reduction of H+ to H2, and (3) the oxidation of H2O to O2.