The interactions of NO with Cu-ZSM-5 have been investigated by means of infrared spectroscopy. Following reduction by CO, most of the copper is present as Cu+ cations, Room-temperature exposure of the reduced catalyst to NO results in the immediate appearance of Cu+(NO) and Cu+(NO)(2), With time, these species disappear and are replaced by Cu2+(NO) and Cu2+(O-)(NO). Evidence of the formation of Cu2+(NO2) and CU2+(NO3-) and of adsorbed N2O and N2O3 is also observed. Similar species are observed upon room-temperature exposure of autoreduced and preoxidized Cu-ZSM-5. Above 573 K, the catalyst is active for NO decomposition to N-2 and N2O. The selectivity to Nz increases rapidly with increasing temperature and is essentially 100% at 773 K, the temperature at which the catalyst exhibits maximum activity. Infrared spectra taken under reaction conditions show weak peaks for Cu+(NO), Cu2+(O-)(NO), and CU2+(NO3-). With increasing temperature, the intensities of the peaks for Cu+(NO) and Cu2+(O-)(NO) decrease but the proportion of the former species increases relative to the latter, Based on this evidence and rate data reported in the literature, a mechanism is proposed for the decomposition of NO, The first step in this mechanism is the formation of N2O via the decomposition of Cu+(NO)(2). N-2 is then formed via the reaction of N2O with Cu+ sites, O-2 formation is envisioned to proceed via the release of O atoms from Cu2+O- and the subsequent reaction of O atoms with additional Cu2+O- to produce Cu(2+)0(2)(-). The variation in the fraction of Cu+ with temperature, deduced from the proposed mechanism, is in qualitative agreement with recent XANES observations.