Co3O4 is a promising catalyst for removing CO from H-2 streams via the preferential CO oxidation (PROX). A Mars-van-Krevelen redox mechanism is often suggested but a detailed knowledge especially of the oxidation state of the catalytically active surface under reaction conditions is typically missing. We have thus utilized operando X-ray absorption spectroscopy to examine structure and oxidation state during PROX, and near atmospheric pressure-XPS at low photoelectron kinetic energies and thus high surface sensitivity to monitor surface composition changes. The rather easy surface reduction in pure CO (starting already at 100 degrees C) and the easy reoxidation by O-2 suggest that molecularly adsorbed CO reacts with lattice oxygen, which is replenished by gas phase O-2. Nevertheless, the steady state concentration of oxygen vacancies under reaction conditions is too low even for XPS detection so that both the bulk and surface of Co3O4 appear fully oxidized during PROX. Furthermore, the effect of adding CeO2 (a less active material) to Co3O4 was studied. Promotion of Co3O4 with 10 wt% CeO2 increases the reduction temperatures in CO and H-2 and enhances the PROX activity. Since CeO2 is a less active material, this can only be explained by a higher activity of the Co-O-Ce interface. (C) 2016 The Author(s). Published by Elsevier Inc.