2.5 nm gold nanoparticles were grown on a fumed silica support, using the physical vapor deposition technique of magnetron sputtering, that are thermally stable when annealed in an oxygen containing environment up to at least 500 degrees C. Traditional Au/TiO2 catalysts rapidly sinter to form large 13.9 nm gold clusters under these annealing conditions. This surprising stability of Au/SiO2 is attributed to the absence of residual impurities (ensured by the halide-free production method) and a strong bond between gold and defects at the silica surface (about 3 eV per bond) estimated from density functional theory (DFT) calculations. The Au/SiO2 catalysts are less active for CO oxidation than the prototypical Au/TiO2 catalysts, however they can be regenerated far more easily, allowing the activity of a catalyst to be fully recovered after deactivation. (C) 2008 Elsevier Inc. All rights reserved.