The strongly acidic Bronsted sites of H-ZSM-5 can be quantitatively exchanged with monovalent copper ions by reaction with CuCl at 573 K, as evidenced by the disappearance of the characteristic IR bands of bridged OH groups. Characterization of the Cu-ZSM-5 samples prepared following this route by means of UV-Vis-NIR (diffuse reflectance) and photoluminescence spectroscopies confirms that the protons are substituted by Cu+ ions, which are isolated and located in a few, structurally well defined sites easily accessible to ligand molecules. These Cu+ ions are highly coordinatively unsaturated and can form Cu+ (CO)n (n = 1, 2 or 3) carbonylic and Cu+ (NO)n (n = 1 or 2) nitrosylic complexes upon dosage of carbon monoxide or nitric oxide at 77 K. The Cu+ (NO)2 dinytrosylic complexes are unstable at room temperature and evolve with formation of nitrous oxide, NO2- and oxidized Cu(II)-NO species. This behaviour strongly supports the hypothesis that a redox mechanism is operating in the nitric oxide decomposition reaction leading to nitrogen and oxygen.