In order to investigate the photocatalytic stability and/or lifetime of titanium oxide photocatalysts in detail, standard reference photocatalysts (JRC-TIO-5 -3, -4, and -5), of which the surface chemical and physical properties are well known, were studied for the photocatalytic decompositon of NO under a large-scale flow reaction system. In the first stage, the photocatalytic decomposition of NO proceeded with a high efficiency and continuous production of N-2, O-2, and N2O. However, the reaction efficiency gradually decreased and finally leveled off at constant values. These values were found to be in the order of JRC-TIO-4 > -3 > -5 > -2. Anatase TiO2, with a large surface area and numerous OH groups, was found to exhibit a high efficiency for the decomposition of NO in the flow system. The optimal pretreatment and reaction conditions were observed at a pretreatment temperature around 573 K under a mixture of O-2 and Ar. It was confirmed that the surface hydroxyl groups on the photocatalysts play an important role in the decomposition of NO under this flow system. (C) 2001 Academic Press.