A combined Fourier transform IR (FT-IR) and electron paramagnetic resonance (EPR) study shows that copper in ’excessively exchanged’ Cu/ZSM-5 is initially present as OH bridged Cu2+ dimers, besides isolated Cu2+ ions. Upon heating, the dimers lose water and become oxygen bridged [Cu-O-Cu](2+) complexes. These are ’EPR-silent’, presumably as a consequence of antiferromagnetic coupling of the unpaired electrons in each Cu2+; they are, however, detectable by their perturbation of the lattice vibrations, detected by a FT-IR band at 918-923 cm(-1). Reduction by hydrogen or carbon monoxide converts the [Cu-O-Cu](2+) complexes to pairs of Cu+ ions, while the color changes from green to grey. Reductive adsorption of nitrogen monoxide on Cu2+ results in the formation of Cu+-NO+. Destructive thermal desorption of nitrogen monoxide at 100 degrees C not only restores the Cu2+ ions, but also appears to regenerate the [Cu-O-Cu](2+) complex. The results suggest that pairs of copper ions are instrumental in the catalytic decomposition of nitrogen monoxide.