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The NH₃-mediated selective catalytic reduction (NH₃-SCR) of NOx over Cu-ion-exchanged chabazite (Cu-CHA) catalysts is the basis of the technology for abatement of NOx from diesel vehicles. A crucial step in this reaction is the activation of oxygen. Under conditions for low-temperature NH₃-SCR, oxygen only reacts with Cu^I ions, which are present as mobile Cu^I diamine complexes [Cu^I(NH₃)₂]⁺. To determine the structure and reactivity of the species formed by oxidation of these Cu^I diamine complexes with oxygen at 200 °C, we have followed this reaction, using a Cu-CHA catalyst with a Si/Al ratio of 15 and 2.6 wt% Cu, by X-ray absorption spectroscopies (XANES and EXAFS) and diffuse reflectance UV-Vis spectroscopy, with the support of DFT calculations and advanced EXAFS wavelet transform analysis. The results provide unprecedented direct evidence for the formation of a [Cu₂(NH₃)₄O₂]²⁺ mobile …