Magnesium oxide was found to show activity for the reduction of carbon dioxide to carbon monoxide under photoirradiation using hydrogen as a reductant. Fourier transform infrared spectroscopy was applied to a study of the reaction mechanism by detection and identiÐcation of the surface species arising during the photoreaction. The formation of surface formate ion was observed during the photoreaction. Since CO was produced from the surface formate in the presence of under irradiation, the surface formate was a CO 2 reaction intermediate which acted as a reductant and converted another molecule to CO. The correlation CO 2 of the reaction activity with the amount of introduced indicated that adsorbed carbonate was reduced by CO 2 to the surface formate, and that the surface formate also reduced the adsorbed carbonate to CO. The IR H 2 spectra showed the di †erence in the adsorption form of between the adsorbed carbonate reduced by CO 2 H 2 to the surface formate and that reduced by the surface formate to CO.