Selective catalytic reduction (SCR) of NO with NH3 in the presence Of 02 takes place over titanium oxide under photoirradiation at low temperature. The active site of TiO2 is suggested to be the Ti4+ Lewis acid site. TiO2 exhibits the weak Lewis acid property. NH3 is adsorbed easily on the Lewis acid site as compared with NO. It is conceivable that NO in the gas phase attacks NH3 adsorbed on the Lewis acid site. N-2 and H2O are generated via a nitrosamide intermediate. Consequently, the reduced Ti3+ species is formed and is reoxidized to the Ti4+ species by oxygen. NO can be also adsorbed directly on the Lewis acid site which is unoccupied by NH3 and transformed to inactive the nitrate species. In the absence of oxygen, NO reoxidizes the Ti3+ species and N2O was produced.