In situ XAFS combined with UV-vis-near-IR spectroscopy are used to identify the active site in copper-loaded ZSM-5 responsible for the catalytic decomposition of NO. Cu-ZSM-5 was probed with in situ XAFS (i) after O-2 activation and (ii) while catalyzing the direct decomposition of NO into N-2 and O-2. A careful R-space fitting of the Cu K-edge EXAFS data is presented, including the use of different k-weightings and the analysis of the individual coordination shells. For the O-2-activated overexchanged Cu-ZSM-5 sample a (CuCu)-Cu-... contribution at 2.87 Angstrom with a coordination number of 1 is found. The corresponding UV-vis-near-IR spectrum is characterized by an intense absorption band at 22700 cm(-1) and a relatively weaker band at 30000 cm(-1), while no corresponding EPR signal is detected. Comparison of these data with the large databank of well-characterized copper centers in enzymes and synthetic model complexes leads to the identification of the bis(mu-oxo)dicopper core, i.e. [Cu-2(mu-O)(2)](2+). After dehydration in He, Cu-ZSM-5 shows stable NO decomposition activity and the in situ XAFS data indicate the formation of a large fraction of the bis(mu-oxo)dicopper core during reaction. When the Cu/Al ratio of Cu-ZSM-5 exceeds 0.2, both the bis(mu-oxo)dicopper core is formed and the NO decomposition activity increases sharply. On the basis of the in situ measurements, a reaction cycle is proposed in which the bis(mu-oxo)dicopper core forms the product O-2 on a single active site and realizes the continuous O-2 release and concomitant self-reduction.