Photocatalytic reduction of NO with CO has been carried out using various single-site photocatalysts under UV-light irradiation. The single-site photocatalysts prepared by incorporating transition metal oxides within SiO 2 (M/SiO 2 : MMo, V, and Cr) display different photocatalytic activities. Mo/SiO 2 promotes photocatalytic reduction of NO with CO efficiently to produce N 2 via the intermediate N 2 O. V/SiO 2 was observed to promote the reaction that leads to selective production of N 2 O that does not undergo subsequent reduction to form N 2 . When Cr/SiO 2 is used as the photocatalyst, reduction of NO does not take place. The results of Fourier transform infrared investigations reveal that differences in photocatalytic activities are due to different reactivity of reactants-adsorbed transition metal oxides in M/SiO 2 and, consequently, the reaction mechanisms for the photocatalytic reduction of NO with CO over M/SiO 2 are proposed.