Adsorption of NO on an over-exchanged Cu-ZSM-5 sample (SiO2/Al2O3 = 23) leads to formation of the well-known nitrosyl species of Cu2+ and Cut. In addition, two kinds of Cu3+-NO mononitrosyls are detected by IR bands at 1960 and 1947 cm(-1). The Cu3+ species are produced as a result of disproportionation of Cu2+ ions from associated sites: Cu2+-O-Cu2+ Cu+-O-Cu3+. The formation of Cu+ ions during NO adsorption is proven with a sample where the preexisting Cut sites were blocked by CO. In the presence of NO, the Cu3+-NO complexes are reduced to Cut-NO and Cu2+-NO. At low temperature, the Cu3+-NO mononitrosyls are able to accept a second NO ligand forming thus Cu3+(NO)(2) species (1915 and 1863-53 cm(-1)). The dinitrosyl structures are proven by adsorption of NO-(NO)-N-15 isotopic mixture. The new findings support the hypothesis that Cut ions are active sites in NO decomposition and the dinitrosyl species, reaction intermediates. (C) 2013 Elsevier Inc. All rights reserved.