We report the use of near-edge X-ray absorption fine structure (NEXAFS) to unambiguously characterize the oxidation state of copper in copper oxides. The method is based on the analysis of the oxygen K-edge and copper L-edge features of Cu 2O and CuO powder materials. To further demonstrate its utility, NEXAFS was also used to follow spectroscopically the oxidation of a Cu(100) single crystal surface by NO 2, and the thermal decomposition of the resulting oxides. In agreement with previous studies [J.A. Rodriguez, J. Hrbek, J. Vac. Sci. Technol. 12 (1994) 2140], our NEXAFS results indicate that NO 2 dissociates on the metal surface, generating NO and atomic oxygen which subsequently reacts with the Cu(100) surface to produce Cu 2O (300 K) or a mixture of Cu 2O and CuO (250 K). Upon heating, the thermodynamically unstable CuO transforms into Cu 2O.