Dinitrogen (N 2 ) adsorption on overexchanged Cu-ZSM-5 (Si/Al ) 25, 104% ion exchange from copper acetate) was studied at 302 K by diffuse reflectance infrared spectroscopy (DRIFTS), N 2 -adsorption isotherms, and temperature-programmed N 2 desorption (TPD). We found that Cu-ZSM-5 adsorbs N 2 at least in two different modes. The weaker mode, evidenced by the appearance of an IR band at 2295 cm -1 , was already reported in the literature and assigned to N 2 coordinated on-top on Cu(I) sites. This kind of adsorption occurs only under N 2 atmosphere and is completely reversible at 302 K. The stronger mode, revealed by TPD and by the difference between total and reversible N 2 -adsorption isotherms, is completely irreversible at 302 K. The band at 2130 cm -1 , previously assigned by us to this kind of strong N 2 adsorption, is not observed if ultrapure gases are utilized. The introduction of controlled amounts of impurities, together with the utilization of 15 N 2 and 13 CO shows that this band is instead related to hydrated Cu(I)-CO species formed by accumulation on the sample of traces of H 2 O in the N 2 flow and from residual CO produced during activation. The coordination of N 2 onto Cu(I)‚‚‚Cu(I) dimers is proposed as an adequate model to describe this IR-silent irreversible N 2 adsorption.