With the aim of understanding the elementary steps governing the oxidation of CO catalyzed by dispersed or supported gold nanoclusters, the adsorption of molecular species, such as O-2 and CO, on model neutral and positively charged clusters (Au-n(m+) n = 1, 9, and 13; m = 0, 1, and 3) has been studied using an ab initio approach. The computed structural and thermodynamic data related to the binding process show that molecular oxygen interacts better with neutral clusters, acting as an electron acceptor, while CO more strongly binds to positively charged species, thus acting as an electron donor.