The role of the bis(mu-oxo)dicopper core, i.e., [Cu-2(mu-O)(2)](2+), in the decomposition of NO and N2O by the Cu-ZSM-5 zeolite has been studied with combined operando UV-vis monitoring of the catalyst and on-line GC analysis. An optical fiber was mounted on the outer surface of the quartz wall of the plug-flow reactor and collected the UV-vis diffuse reflectance spectra under true catalytic conditions. Measurement under transient reaction conditions indicated that [Cu-2(mu-O)(2)](2+) is formed by O abstraction of N2O, which is an intermediate in the NO decomposition. This conversion of N2O to N-2 and O-2 is strongly retarded below 673 K. Above 673 K, the produced [Cu-2(mu-O)(2)](2+) fulfills the role of O-2 release, guaranteeing the self-reduction of the catalytic site. Studying the NO decomposition as a function of the O-2 content in the feed strongly suggested that O-2 release from [Cu-2(mu-O)(2)](2+) is rate limiting in the NO decomposition at 773 K. (C) 2003 Elsevier Inc. All rights reserved.