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 N-15(2) and (CO)-C-13 shows that this band is instead related to hydrated Cu(I)-CO species formed by accumulation on the sample of traces of H2O 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.