An FTIR study of CO adsorption from 120 K up to room temperature on a series of Au-ceria samples is presented. Samples with low gold content (0.7 and 0.6 at%) were prepared by urea gelation/co-precipitation and by cyanide leaching of the high-gold content (5.8 at%) material prepared by deposition-precipitation on La-doped CeO2. The samples were subjected to different pretreatments to collect information on the surface composition under working conditions. An absorption band at 2130-2140 cm(-1), not reversible on outgassing and more resistant to oxidation than the usual carbonyl band on Au-0 sites, was present due to CO adsorbed on cationic gold clusters. This highly stable species is relevant for hydrogen gas upgrade by removing CO from reformate-type gases at low temperatures. In addition, a broad absorption band in the 2000-2100 cm(-1) range was observed after reduction in hydrogen, due to structural and electronic changes of gold. Interestingly, the reduced gold species in ceria. can be reoxidized at mild conditions. Light-off of the CO oxidation reaction took place below room temperature on the metallic gold-containing ceria but was delayed until 310 K on the ionic gold-containing sample. TPR and XPS analysis of the fresh and used catalysts corroborated the stability of ionic gold in ceria up to 393 K in the reaction gas mixture. (c) 2006 Elsevier Inc. All rights reserved.