Nanocrystalline CeO2 was prepared by precipitation of a solution of the acetate using supercritical CO2 as an antisolvent. It was demonstrated that gold supported on this material is very active for the oxidation of CO at ambient temperature, particularly in comparison with CeO2 prepared in a conventional manner by thermal decomposition of the acetate. Comparing the catalytic performance for CO oxidation with the most active catalysts in the current literature confirms the high activity of these new materials. They are considerably more active than previous Au/CeO2 catalysts. The catalyst activity was found to be dependent on the precipitation conditions, which in turn was found to influence the dispersion of gold on the support, as evidenced by detailed microscopy and spectroscopy characterization. The most active fresh catalyst exhibited highly dispersed gold and showed no evidence of the existence of An nanocrystals using detailed STEM analysis. Following reaction with CO/O-2, subtle microstructural changes were apparent, although the morphology of the nanocrystalline CeO2 support was unchanged; in particular, the An, which was previously uniformly dispersed, showed signs of beginning to agglomerate into sub-5 nm particles. The stability and origin of the catalytic activity are discussed. (c) 2007 Elsevier Inc. All rights reserved.