Gold nanoparticles smaller than similar to 2 nm show interesting catalytic properties arising from their size-determined electronic and geometrical structures. They are able to dissociate molecular O(2) with low activation barriers and become oxidized in a reversible way at low temperature. Several oxygen species adsorbed on the nanoparticle surface, as well as different gold sites including low coordinated neutral Au(0) atoms and cationic Au(I) and Au(delta+) species, are generated on the oxidized nanoparticles. The implication of these new sites in the mechanism of different gold-catalyzed reactions in which oxygen plays a crucial role is discussed here. (C) 2011 Elsevier Inc. All rights reserved.