The active redox oxygen site of a mixed copper cerium oxide catalyst (Cu0.07Ce0.93O2) and the active copper species for CO preferential oxidation in rich H-2 were investigated by in situ X-ray absorption spectroscopy (XAS) at Cu K-edge during anaerobic reaction along with X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), ultraviolet-visible-near infrared (UV-Vis-NIR) diffuse reflectance spectroscopy. During anaerobic reaction, the absorption edge energy of in situ X-ray absorption near edge spectroscopy (XANES) spectra shifts to lower energy and the white-line intensity decreases, suggesting the reduction of Cu(II) to Cu(I) via the removal of surface lattice oxygen to oxidize CO. In addition, the first shell Cu-O peak intensity decrease of a series of Fourier transformed extended X-ray absorption fine structure (EXAFS) spectra during anaerobic reaction indicates the loss of copper's first oxygen neighbor to oxidize CO. These provided for the first time the experimental evidence that the active/labile oxygen in copper cerium oxide catalysts is the first oxygen neighbor of Cu(II) species, which is further supported by UV-Vis studies on the reduction of the optical absorption edge energy of copper in the catalyst. (C) 2012 Elsevier Inc. All rights reserved.