CuOx/CeO2 nanocatalysts were synthesized using the silica aquagel co-precipitation (SACOP) technique and tested for the preferential oxidation of CO (PROX). In spite of their high catalytic activity and selectivity, it was found that deactivation restricted their utilization in practical conditions, as is the case for most copper-based catalysts analyzed in the PROX process. in order to determine the causes of deactivation exhaustive analyses of the evolution of the composition of the catalyst surface during the reaction were performed by means of spectroscopic techniques (XPS and FTIR). Moreover step-response experiments under different atmospheres were carried out in order to establish the deactivating potential of the different gases present in the reactant stream. The deactivation mechanism comprises the formation of surface carbonates (the main cause of deactivation) and the partial sintering of copper (by the aggregation of dispersed Cu2+ in CuO clusters), both of which are a function of the gas concentration, temperature and reaction time. (C) 2009 Elsevier B.V. All rights reserved.