This study examined the oxidation and reduction behavior of mass-selected Au clusters consisting of 2-13 atoms deposited on silica. An atomic oxygen environment was used for the oxidation of Au. X-ray photoelectron spectroscopy (XPS) was used to identify Au(III) and Au(O). Au-5, Au-7 and Au-13 clusters deposited on the as-prepared SiO2/Si substrates were highly inert towards oxidation, whereas the other clusters could be oxidized, i.e. the chemical property drastically changed with the number of atoms in a cluster. The size-selectivity in chemical reactivity remained unchanged upon air-exposure. The chemical properties of the deposited Au clusters were unchanged after annealing at 250 degrees C. Annealing at higher temperatures caused structural changes to the surface, as determined by the oxidation behavior. XPS of the deposited Au clusters upon annealing indicated charge transfer from Au to silica. (C) 2009 Elsevier B.V. All rights reserved.