One of the challenges of catalysis is the transformation of inert C−H bonds to useful products. Copper-containing monooxygenases play an important role in this regard. Here we show that low-temperature oxygenation of dinuclear copper(I) complexes leads to unusual tetranuclear, mixed-valent μ4-peroxo [CuI/CuII]2 complexes. These Cu4O2 intermediates promote irreversible and thermally activated O−O bond homolysis, generating Cu2O complexes that catalyze strongly exergonic H-atom abstraction from hydrocarbons, coupled to O-transfer. The Cu2O species can also be produced with N2O, demonstrating their capability for small-molecule activation. The binding and cleavage of O2 leading to the primary Cu4O2 intermediate and the Cu2O complexes, respectively, is elucidated with a range of solution spectroscopic methods and mass spectrometry. The unique reactivities of these species establish an unprecedented, 100 % atom-economic scenario for the catalytic, copper-mediated monooxygenation of organic substrates, employing both O-atoms of O2.