The chemical structure of a Cu(111) model catalyst during the CO oxidation reaction in the CO+O-2 pressure range of 10-300 mTorr at 298-413 K was studied in situ using surface sensitive X-ray photoelectron and adsorption spectroscopy techniques [X-ray photoelectron spectroscopy (XPS) and near edge X-ray adsorption fine structure spectroscopy (NEXAFS)]. For O-2:CO partial pressure ratios below 1:3, the surface is covered by chemisorbed O and by a thin (similar to 1 nm) Cu2O layer, which covers completely the surface for ratios above 1:3 between 333 and 413 K. The Cu2O film increases in thickness and exceeds the escape depth (similar to 3-4 nm) of the XPS and NEXAFS photoelectrons used for analysis at 413 K. No CuO formation was detected under the reaction conditions used in this work. The main reaction intermediate was found to be CO2 delta-, with a coverage that correlates with the amount of Cu2O, suggesting that this phase is the most active for CO oxidation.