The adsorption of nitrogen oxides on Cu-ZSM-5 was studied by infrared spectroscopy to elucidate the species associated with the band at 2133 cm(-1). The band was found for both NO and NO2 adsorption. Labeling experiments with (NO)-N-15 revealed that the associated surface species contained nitrogen and, most likely, an N-O bond. Go-adsorption experiments of NO and oxygen produced adsorbed nitronium, NO2+, as the principal, associated species. Adsorption of nitrogen oxides on dispersed CuO and the HZSM-5 support demonstrated that the 2133 cm(-1) band was not necessarily associated with copper ions. A relatively strong correlation between the bands at 2133 and 3615 cm(-1) indicates that the primary adsorption sites of NO2+ are the strongly protic, bridging Si(OH)Al framework hydroxyls. Once these were filled, other, weaker acid sites began to adsorb NO2+.