We have investigated the adsorption of O2 and CO on neutral Au-n clusters, by means of first-principles density functional calculations. The O-2 adsorption energy shows odd-even effects. The adsorption of CO occurs on top of the least coordinated Au atom, except for Au-5 and Au-7 where the bridge position is preferred. In these bridge sites, both the CO electronic charge and bond distance increase, whereas the vibrational frequency decreases, as compared with the values for CO adsorbed on top sites. By analyzing the partial density of states we show that the enhancement of pi back-donation in bridge isomers is the cause of these trends. (c) 2005 Elsevier B.V. All rights reserved.