The objective of the present study is to develop the reaction mechanism and kinetics of photoreduction of NO by CO. For this purpose, the reactions were conducted in the presence of Pd-ion-substituted nano-TiO2Ti 1-xPdxO2-δ, which was synthesized via a solution combustion method. The photocatalytic activity was investigated with unsubstituted TiO2,1% Pd/TiO2(imp), and Ti 1-xPdx)2-δ (where x = 0.05-0.3). No appreciable NO conversion was observed over unsubstituted TiO2, although, despite competitive adsorption of NO and CO on the Pd2+ sites, there was a significant reduction of NO over Ti1-xPd x)2-δ The kinetic model showed that the enhanced catalytic activity is due to the NO photodissociation at the oxide-ion vacancy. © 2007 American Chemical Society.