We study the effect of gold doping on oxygen vacancy formation and CO adsorption on the (110) and (100) Surfaces of ceria by using density functional theory, corrected for on-site Coulomb interactions (DFT + U). The Au dopant substitutes a Ce atom in the surface layer, leading to strong Structural distortions. The formation of one oxygen vacancy near a dopant atom is energetically "downhill" while the formation of a second vacancy around the same dopant requires energy. When the surface is in equilibrium with gaseous oxygen at I arm and room temperature there is a 0.4 probability that no oxygen atom left the neighborhood of a dopant. This means that the sites where the dopant has not lost oxygen are very active in oxidation reactions. Above 400 K almost all dopants have an oxygen vacancy next to them and an oxidation reaction in such a system takes place by creating a second vacancy. The energy required to form a second vacancy is smaller on (110) than on (100). On the (110) surface, it is much easier to form a second vacancy on the doped Surface than the first vacancy on the undoped surface. The energy required to form a second oxygen vacancy on (100) is comparable to that of forming the first vacancy on the undoped Surface. Thus doping makes the (110) surface a better oxidant but it has a small effect on the oxidative power of the (100) surface. On the (110) surface CO adsorption results in formation of a Carbonate-like structure, similar to the undoped surface, while on the (100) surface direct formation of CO2 is observed, in contrast to the undoped surface. The Au dopant weakens the bond of the surrounding oxygen atoms to the oxide making it a better oxidant, facilitating CO oxidation. (C) 2008 Elsevier B.V. All rights reserved.