The effect of CO2 and H-2 on the activity and deactivation of Au/CeO2 catalysts in the low-temperature water-gas shift reaction was investigated by combined kinetic and in situ IR spectroscopic measurements, going stepwise from idealized to realistic reaction atmospheres. The variation of the reaction atmosphere results in considerable variations in the initial activity and deactivation behavior, with significantly lower activity in the presence of H-2 and more pronounced deactivation in CO2-rich gases. In situ DRIFFS measurements reveal a complex interaction of the different components in the formation of adsorbed reaction intermediates and side products. The bidentate formate species (1588 and 2833 cm(-1)), which are dominant in H-2-free atmospheres and act as reaction intermediates, are replaced by less reactive bidentate (1560 and 2858 cm(-1)) and bridge-bonded (2933 and 2961 cm(-1)) formates in H-2-rich atmospheres. Deactivation is related mainly to the formation of thermally stable monodentate carbonates, which act as reaction-inhibiting spectator species. Their formation is enhanced by CO2 in the reaction gas, whereas H2 leads to their reactive removal by reaction to formates. Possible mechanisms for the deactivation process are discussed. (c) 2006 Elsevier Inc. All rights reserved.