Abstract Copper ion exchanged H-mordenite has been studied with X-ray absorption spectroscopy together with temperature-programmed desorption of NH3 and NO, and its catalytic activity examined in order to elucidate the role and the structure of copper ion in selective catalytic reduction of NO by NH3. Addition of copper to H-mordenite greatly improved the activity of NO reduction by increasing the adsorption capacity of both NH3 and NO. X-ray absorption spectroscopy indicated the formation of a copper—amine compound which could be an intermediate of the reaction. Upon addition of SO2 to the reaction mixture, NO conversion decreased rapidly with time on stream. This deactivation was correlated with the disappearance of the amine intermediate, as observed by X-ray absorption. There was no evidence of the formation of any stable copper sulfate.