The effect of photoirradiation on adsorption of NO on ZnO powder was examined at 303 K, and the photoconductivity change of ZnO during adsorption of NO was also determined. Photoirradiation gave rise to a marked chemisorption of NO on ZnO. The photochemisorption led to an initial decrease and the following increase in the photoconductivity. In situ FT-IR examination showed the formation of oxidized species of NO upon photochemisorption. These results indicate two processes of the trapping of a photogenerated electron by the chemisorbed NO and the pseudo-oxygen defect formation on oxidation of NO.