Abstract A study is made of the chemisorption of CO and SO 2 over Cu-mordenite by means of electron spin resonance (ESR). Two Cu 2+ magnetic complexes are observed in Cu-mordenite, and the reaction with CO causes a differential reduction in their respective line intensities. For reaction at room temperature, the effect is reversible upon removal of the CO, but when the system is heated the catalyst changes irreversibly. Introduction of SO 2 to the Cu-mordenite-CO system causes the reappearance of the Cu 2+ spectrum in a single, different, magnetic complex. Models are proposed to explain the chemical processes operative in the reactions.