The effects of presence of NO2 or water vapor on the enhancement of the amount of adsorption of NO are investigated using a fixed bed of hydrogen-type synthetic mordenite at 303 K. The amount of NO adsorbed increases with increasing concentration of NO2 in the mixed gas including 400 ppm NO, and it also increases with increasing concentration of NO in the mixed gas including 300 ppm NO2: the presence of NO2 enhances the amount of NO adsorbed on the hydrogenated synthetic mordenite. Meanwhile, the presence of water vapor causes a decrease in the amount of NO adsorbed.