To evaluate the fundamental process Of O-2 activation at a single copper site that occurs in biological and catalytic systems, a detailed study Of O-2 binding to Cu(l) complexes of beta-diketiminate ligands L (L-1 = backbone Me; L-2 = backbone tBu) by X-ray crystallography, X-ray absorption spectroscopy (XAS), cryogenic stopped-flow kinetics, and theoretical calculations was performed. Using synchrotron radiation, an X-ray diffraction data set for (LCuO2)-Cu-2 was acquired, which led to structural parameters in close agreement to theoretical predictions. Significant Cu(Ill)-peroxo character for the complex was corroborated by XAS. On the basis of stopped-flow kinetics data and theoretical calculations for the oxygenation of (LCu)-Cu-1(RCN) (R = alkyl, aryl) in THF and THF/RCN mixtures between 193 and 233 K, a dual pathway mechanism is proposed involving (a) rate-determining solvolysis of RCN by THF followed by rapid oxygenation of (LCu)-Cu-1(THF) and (b) direct, bimolecular oxygenation of (LCu)-Cu-1(RCN) via an associative process.