The aqueous electrochemistry of ultrathin silica films supported on metal substrates has been investigated in an apparatus that combines an ultrahigh vacuum analysis chamber with an electrochemical cell. The silica films were prepared in vacuum by evaporating silicon in oxygen ambient onto a platinum substrate. It is established that the silica films completely cover the substrate and isolates it from the solution. The silica films on platinum are stable under electrochemical conditions of cycling. It is shown that the thin silica films (15-100 angstrom) are electrically conductive, with a conductivity several orders of magnitude larger than bulk silica. The silica/Pt electrodes behave over a wide range of potentials as ideally polarizable, which facilitate electrochemical studies of supported metal particles of Pd and Pt. The under-potential deposition of copper onto silica-supported palladium particles and methanol oxidation on platinum particles have also been demonstrated.