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New information about the active sites for the water gas shift (WGS) reaction over Cu-CeO 2 systems was obtained using in-situ, time-resolved X-ray diffraction (TR-XRD), X-ray absorption spectroscopy (TR-XAS, Cu K and Ce L 3 edges), and infrared spectroscopy (DRIFTS). Cu-CeO 2 nanoparticles prepared by a novel reversed microemulsion method (doped Ce 1-x Cu x O 2 sample) and an impregnation method (impregnated CuO x / CeO 2 sample) were studied. The results from all of the samples indicate that both metallic copper and oxygen vacancies in ceria were involved in the generation of active sites for the WGS reaction. Evidence was found for a synergistic Cu-O vacancy interaction. This interaction enhances the chemical activity of Cu, and the presence of Cu facilitates the formation of O vacancies in ceria under reaction conditions. Water dissociation occurred on the O vacancy sites or the Cu-O vacancy interface. No significant amounts of formate were formed on the catalysts during the WGS reaction. The presence of strongly bound carbonates is an important factor for the deactivation of the catalysts at high temperatures. This work identifies for the first time the active sites for the WGS reaction on Cu-CeO 2 catalysts and illustrates the importance of in situ structural studies for heterogeneous catalytic reactions.