The catalytic reduction of NO2 with C3H6 in the presence of O-2 was studied over CuO and Cu/mordenite (Cu/M) by the use of the temperature programmed desorption (TPD) and the temperature programmed reduction (TPR) methods. It was found that the reduction of NO2 with C3H6 over CuO was greatly suppressed in the presence of O-2, whereas over Cu ion exchanged mordenite the conversion of NO2 to N-2 was enhanced by the presence of O-2. It was suggested that NO3 type adsorbed species were effectively involved in the reaction over Cu/M.