Copper ions on over-exchanged Cu-MCM-22 were monitored by CO and NO adsorption at 300 K and NO adsorption at 77 K. Two different species of both Cu(I) and Cu(II) ions were produced by vacuum thermal treatment at 823 K. Cu-I-CO complexes were formed upon CO adsorption and converted to Cu-I-(CO)(2) complexes upon increasing the CO pressure. CO adsorbed on copper ions on small oxide aggregates or in oligomeric cationic compounds was also found. Mononitrosylic Cu-I-NO complexes (band at 1813 cm(-1)) were formed upon NO adsorption and completely oxidized to Cu-II-NO complexes (absorptions at 1899 and 1908 cm(-1)) at 300 K upon increasing NO pressure. Nitro, nitrate, and nitrito species were formed by reaction of NO at 300 K with oxidic surface aggregates. Low-temperature NO adsorption experiments inhibited both the Cu(I) to Cu(II) conversion and the reactivity of the aggregates and offered clear-cut evidence of the presence of two Cu(I) sites in the MCM-22 zeolite (IZA code, MWW) structure. Besides, under higher NO pressure, mononitrosylic Cu-I-NO complexes transform to dinitrosylic Cu-I-(NO)(2) complexes. Dimers of NO, as free species or bound to copper ions, we I re also observed and formed probably by NO confinement within the small spaces of the MWW cages at very low temperatures. The peculiar reactivity Of Cu-MCM-22 under NO adsorption at 300 K suggests that this material is a potential catalyst for NOx abatement.