The atomic structures of gold supported on (1 1 1) and (1 1 0) surfaces of CeO2 have been studied using density-functional theory calculations. A single Au atom is placed on three adsorption sites on the surfaces; the stoichiometric surfaces, an oxygen vacancy and a Ce-vacancy. It is found that (i) the Au adsorption energies are in the following order: Ead(Ce-vacancy) > Ead(O-vacancy) > Ead(stoichiometric surface); and (ii) the Au atom adsorption on the Ce-vacancy activates O atoms nearby. One O atom is less stable than that in O2 in the gas phase and another O atom is much easier to remove compared to that of the stoichiometric surfaces. These results suggest that the Au adsorption on Ce-vacancies not only creates an O-vacancy but also activates an O atom nearby. This provides a piece of direct evidence that Au adsorption on a Ce-vacancy may be responsible for some unique catalytic properties of Au/CeO2.