Supported Au nanoclusters synthesized from diblock copolymer micelles can be reliably prepared with well-controlled sizes and dispersions. For particles with diameters between approximately 1 and 6 nm, the particle size and the support were found to strongly influence the oxygen reactivity, the formation and stabilization of a metal-oxide, and the catalytic activity for electrooxiclation of carbon monoxide. The smallest particles studied (1.5 nm) were the most active for electrooxiclation of CO and had the largest fraction of oxygen associated with gold at the surface as measured by the Au3+/Au-0 X-ray photoemission intensities. Conducting and semiconducting substrates, ITO-coated glass and TiO2, respectively, were associated with greater stabilization of Au3+ oxide as compared to insulating, SiO2, substrates.