There is a need for advanced, corrosion-resistant electrocatalyst support materials for use in fuel cells. To this end, electrically conducting diamond powder was prepared by depositing a layer of boron-doped nanocrystalline diamond on 100 and 500 nm diam diamond powders. The doped layer was deposited by microwave plasma-assisted chemical vapor deposition using an Ar-rich CH4/H-2/Ar/B2H6 source gas mixture. After coating, the 100 nm doped diamond powder had a specific surface area of 27 m(2)/g and an electrical conductivity of 0.41 S/cm. The 500 nm doped diamond powder had a specific surface area of 8 m(2)/g and an electrical conductivity of 0.59 S/cm after coating. The specific surface area of both powders decreased by ca. 50% after diamond coating due mainly to particle-particle fusion. The electrical measurements provided conclusive evidence for a doped diamond overlayer as the uncoated powders possessed no electrical conductivity. Furthermore, the fact that the electrical properties were unaltered by acid washing confirmed that the conductivity arises from the doped diamond overlayer and not any adventitious sp(2) carbon impurity on the particle surface, which is removed by such chemical treatment. Scanning electron microscopy images and Raman spectroscopy yielded further evidence in support of a nanocrystalline diamond overlayer. Both powders exhibited electrochemical responses for Fe(CN)(6)(3-/4-), Ir(Cl)(6)(-2/-3), and Fe+2/+3 that were comparable to typical responses seen for high-quality, boron-doped nanocrystalline diamond thin-film electrodes. The electrochemical behavior of the powders was assessed using a pipette electrode that housed the packed powder with no binder. The 100 nm doped diamond powder electrodes were more plagued by ohmic resistance effects than were the 500 nm powder electrodes because of reduced particle contact. Importantly, it was found that the doped diamond powder electrodes are dimensionally stable and corrosion-resistant during anodic polarization at 1.4 V vs Ag/AgCl (1 h) in 0.5 M H2SO4 at 80 degrees C. In contrast, glassy carbon powder polarized under identical conditions underwent significant microstructural degradation and corrosion. (C) 2008 The Electrochemical Society.