Ab initio density functional theory simulations are performed for carbon monoxide oxidation at gold nano-catalysts, focussing in the influence of multiply-adsorbed CO on the reaction mechanisms. Both small nano-clusters (Au-4, Au-5) and a model of gold nanoparticle edge sites are used. For Au5, CO displaces pre-adsorbed molecular O-2, although oxygen remains close and eventually reacts with CO forming OCOO radicals. Interaction with neighboring co-adsorbed CO then directly forms two CO2 molecules. Similar results are found for gold nanoparticles, although much weaker O-2 binding results in smaller activity. Cooperation of several co-adsorbed entities is therefore an important issue on the mechanisms of oxidation reactions on gold in the low temperature regime. (C) 2008 Elsevier B. V. All rights reserved.