MgO exhibits activity for the reduction of CO2 to CO under photoirradiation in the presence of H-2 or CH4 as a reductant, although MgO is an insulating material. The present study clarified the mechanism of the CO2 photocatalytic reduction in the presence of H-2 or CH4 over MgO. The electron paramagnetic resonance (EPR) spectra show that a CO2 molecule adsorbed on MgO was activated to a CO2- radical under photoirradiation. In addition, it was confirmed by photoluminescence that new acceptor level built up between the valence band and the conduction band of MgO on CO2-adsorbed MgO. The CO2- radical was reduced to a surface bidentate formate or a surface bidentate acetate by H-2 or CH4 in the dark, respectively. The surface bidentate formate anchors on MgO as a photoactive species and reduces CO2 in the gas phase to CO since the CO,, photocatalytic reduction proceeded over MgO absorbing HCHO or CH3CHO and only (CO)-C-12 was formed in the presence of (CO2)-C-12 over MgO modified by a C-13-labeled formate under irradiation. The active species was generated from the side-on adsorption-type bidentate carbonate selectively, although the two types of bidentate carbonates were detected by Fourier transform infrared (FT-IR) spectroscopy. On the other hand, the role of the surface bidentate acetate is under discussion. It is the first report that the substrate-modified insulating material exhibits activity in the CO2 photocatalytic reduction.