CeO2 showed high adsorptivity (4.9 molecules/nm(2)) for NO at 303 K. The adsorbed species of NO on CeO2 in the dark were the NO dimer and/or dinitrosyl, nitrosyl, and NO2-. About 40% of the NO adsorbed in the dark was chemisorbed mainly as NO2-. N2O seemed to be liberated into a gas phase via the intermediate species, such as the NO dimer. The UV irradiation to CeO2-NO systems at 303 Ii induced the photoadsorption of NO and the photocatalytic formation of N2O. Most of the photoadsorbed species were chemisorbed, mainly as photooxidized species, NO2- and NO3-; all other photoadsorbed species were reversibly adsorbed as nitrosyl and dinitrosyl. A frequency shift was observed in a few IR bands of the adsorbed NO, which might have arisen from the photoreduction of Ce4+ to Ce3+.