Reduction of NO 2 with CO in the presence of NO and excess oxygen, a model mixture for flue gas, over a 0.1% Pt/SiO 2 catalyst was studied. The related reaction mechanisms, such as oxidation of CO and NO, were discussed. It was found that there was a narrow temperature window (180-190 ℃) for the reduction of NO 2 by CO. When the temperature was lower than the lower limit of the window, the reduction hardly occurred, while when the temperature was higher than the upper limit of the window, the direct oxidation of CO by O 2 occurred and thereby NO 2 could not be effectively reduced by CO. The presence of NO shifted the window to higher temperatures owing to the inhibition effect of NO on the activation of O 2 on Pt, which made it possible to reduce NO 2 by CO in flue gas.