A catalytic electrode was prepared using a new electrically conducting and corrosion resistant carbon support material, boron-doped diamond. Fabrication of the composite electrode involves a three-step process: (i) continuous diamond thin-film deposition on a substrate, (ii) electrodeposition of Pt catalyst particles on the diamond surface, and (iii) short-term diamond deposition to entrap the metal particles into the surface microstructure. The process results in a conductive, morphologically, and microstructurally stable composite electrode containing metal particles of somewhat controlled composition, size, and catalytic activity. The metal catalyst particles were galvanostatically deposited from a K2PtCl6/HClO4 solution, with the metal particle size (50-350 nm) and distribution (similar to10(9) cm(-2)) being controlled by adjusting the galvanostatic deposition and secondary diamond-growth conditions. For a 300 s Pt deposition time, the estimated loading was 75.8 mug/cm(2), assuming a 100% current efficiency. The composite electrode was extremely stable, both structurally and catalytically, during a 2 h polarization in 85% H3PO4 at 170degreesC and 0.1 A/cm(2). The electrode's catalytic activity was evaluated using the O-2 reduction reaction at room temperature in 0.1 M solutions of H3PO4, H2SO4, and HClO4. The kinetic parameters (Tafel slope and exchange current density) were obtained by cyclic voltammetry and were found to be comparable to those for a polycrystalline Pt electrode in the same media. Tafel slopes of 263 to -80 mV/dec were observed at low overpotentials, with the lowest slope in HClO4 and highest in H3PO4. The exchange current density ranged from 10(-12) to 10(-10) A/cm(2), and increased in the order of H3PO4 < H2SO4 < HClO4. The potential advantages of the composite electrode, as compared with commercial sp(2) carbon electrodes, are (i) the corrosion resistance of the diamond support, resulting in highly stable reaction centers at high potentials, current densities, and temperatures, and (ii) the fact that all of the catalyst particles are strongly anchored at the film surface and are not contained inside pores.