The natures of the surface species formed upon adsorption of NO on copper-ion exchanged mordenite catalysts were investigated by means of temperature programmed desorption (TPD) and diffuse reflectance infrared spectroscopy. When nitrogen monoxide (NO) was contacted with Cu/mordenite at room temperature, the evolution of dinitrogen monoxide (N2O) and nitrogen occurred, accompanied by the formation of the adsorbed oxygen species (Os). In the TPD runs, three NO desorption peaks appeared, with maxima at 383 K (alpha-NO peak), 523 K (delta-NO peak), and 673 K (gamma-NO peak). The desorptions of oxygen and nitrogen dioxide (NO2) were observed together with the desorption of NO at 673 K. These alpha-NO and beta-NO peaks correspond to the desorptions of nitrogen oxide species which adsorbed as NO-type species. The desorptions of gamma-NO, gamma-NO2 and gamma-O-2 at 673 K arose from the decomposition of NO3-type adsorbed species. The NO3-type species may have formed by the reaction between NO and the adsorbed oxygen species (Os). By comparison of the :ate of the NO decomposition with that of the decomposition of NO3-type adsorbed species, it was shown that the NO3-type adsorbed species plays a pivotal role in the decomposition of NO as intermediate of the reaction.