83% of NO conversion and 96% of N-2 selectivity were achieved in the photo-SCR system over TiO2 photocatalyst. N-2 was evolved in the gas phase when NO was flowed over TiO2 pretreated by NH3 under photoirradiation. The photo-SCR reaction proceeds according to Eley-Rideal mechanism. NO in the gas phase attacks NH3 adsorbed on Lewis acid site. Strength and/or amount of the Lewis acid site related to NH3 adsorption had an influence on the N-2 evolution. Therefore, the surface acid property of TiO2 is an important factor to control the N-2 evolution. In addition, the reaction orders of gases (NO, NH3 and O-2) and light intensity were determined by the kinetic studies. The results supported the reaction mechanism which we have proposed in the previous works. Time-determining step of the photo-SCR over TiO2 is the process of decomposition of a NH2NO intermediate under excess O-2 concentration or re-oxidation of Ti3+ to Ti4+ below 2% O-2 concentration. (C) 2004 Elsevier B.V. All rights reserved.