In Au/CeO2 catalysts prepared by the deposition-precipitation method, many Au particles smaller than 3 nm in diameter were clearly observed by annular dark field scanning transmission electron microscopy (ADF-STEM). The water gas shift activity of Au/CeO2 was compared with that of Pt/CeO2, Au/TiO2, and Cu/ZnO/Al2O3 by using temperature-programmed reaction (TPRe) measurement in a stream of a reaction gas mixture composed of CO, CO2, H-2, H2O, and He. Au/CeO2 was found to be the most active at temperatures between 373 and 523 K without producing methane below 623 K. Based on transient experiments consisting of injecting CO and H2O pulses into H-2 and He streams, it is deduced that the water gas shift reaction proceeds over the perimeter interfaces of small gold particles on a reduced cerium oxide surface. The reaction proceeds by formation of a reaction intermediate (possibly a formate) from CO and OH groups, followed by decomposition of the intermediate by H2O into CO2 and H-2. Hydrogen in the reactant gas facilitates cerium oxide to form more active reduced surfaces. (c) 2005 Elsevier B.V. All rights reserved.