Oxygen molecules do not adsorb on flat gold surfaces at room temperature, but recent experimental results have demonstrated adsorption of oxygen on small, negatively charged gold nanoparticles. Here we report first-principles density functional calculations of oxygen adsorption on neutral and charged An(n) nanoparticles. We find that: (i) Oxygen molecules chemisorb on An(n) clusters, with typical binding energies of 0.5-1.5 eV; the binding energy is larger for negatively charged clusters. (ii) In the most stable configurations, the two oxygen atoms are spatially separated and form a linear O-Au-O bridge. The results establish a correlation between the experimental reactivity of gold clusters and the oxygen binding energy. (C) 2003 Elsevier Science B.V. All rights reserved.