The reaction pathways for CO oxidation over anionic gold hexamer(Au-6(-)) as a model catalyst were studied using unrestricted density functional theory (UDFT). The reaction mechanisms for two possible initial structures, in which either CO or O-2 was adsorbed on the apex site while the other was on the nearest lateral site, were analyzed. The reaction proceeds through a typical Langmuir-Hinshelwood mechanism taking place in two steps for both initial structures. First, CO reacts with the molecular O-2 leading to the first CO2 molecule and Au-O complex through the formation of a four-centered intermediate structure (CO-OO). This is followed by the elimination of the remaining O on the cluster by another CO. It was found that the energy barriers for the formation of the intermediate and its decomposition into CO2 and Au-O complex were much lower when O-2 was bonded to the apex site. The second step, on the other hand, was found to be identical for both initial structures. (C) 2009 Elsevier B.V. All rights reserved.