CeO2-supported gold synthesized from Au(CH3)(2)(acac) (acac: acetylacetonate) catalyzed CO oxidation at 353 K, with a turnover frequency TOF of 6.3 x 10(-3) molecules of CO (Au atoms)(-1) at CO and O-2 partial pressures of 1.0 kPa; the apparent activation energy was 138 +/- 2 kJ mol(-1). The activity increased as the catalyst functioned in a flow reactor, and after 48 h on stream, the TOF at 298 K and the aforementioned CO and O-2 partial pressures was (5.6 +/- 0.2) x 10(-2) molecules of CO (Au atom s)(-1). X-ray absorption spectra, reported separately, showed that the catalyst structure changed during operation. as the mononuclear cationic species present initially formed clusters consisting on average of roughly 15 Au atoms each. The gold clusters were more active for CO oxidation catalysis than the mononuclear gold species. Reaction orders of CO oxidation catalyzed by the sample containing the clusters at 303 K were found to be 0.19 in CO, 0.18 in O-2, and -0.4 in CO2. The apparent activation energy characterizing the catalyst containing the clusters was found to be only about one-third of that characterizing the catalyst incorporating the mononuclear cationic gold. (C) 2008 Elsevier Inc. All rights reserved.