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The role of oxygen in the activation of C-H bonds in methane on clean and oxygen-precovered Cu(111) and Cu 2 O(111) surfaces was studied with combined in situ near-ambient-pressure scanning tunneling microscopy and X-ray photoelectron spectroscopy. Activation of methane at 300 K and "moderate pressures" was only observed on oxygen-precovered Cu(111) surfaces. Density functional theory calculations reveal that the lowest activation energy barrier of C-H on Cu(111) in the presence of chemisorbed oxygen is related to a two-active-site, four-centered mechanism, which stabilizes the required transition-state intermediate by dipole-dipole attraction of O-H and Cu-CH 3 species. The C-H bond activation barriers on Cu 2 O(111) surfaces are large due to the weak stabilization of H and CH 3 fragments.