Boron-doped polycrystalline and near-single-crystal quality diamond electrodes were studied by voltammetry. A redox couple with E-1/2 = +1.83 V vs. standard hydrogen electrode was detected on the polycrystalline electrodes but was absent on the single-crystal electrodes. The results strongly suggest that the couple is associated with reactivity at the grain boundaries. Plasma fluorination of polycrystalline diamond electrodes using CF4 in a radio frequency plasma eliminated the redox couple at +1.83 V but did not alter the potential range of water stability. Cathodic polarization of as-grown, polycrystalline diamond electrodes caused an irreversible addition of oxygen to the surface. Subsequent anodic polarization added additional oxygen and made the surface hydrophilic. Single-crystal electrodes also displayed an increase in oxygen coverage upon both cathodic and anodic polarization. Voltammetry studies of electrodes covered with a thin sp(2) carbon surface layer indicate that the redox couple at +1.83 V corresponds to multiple processes including the etching of sp(2) carbon in the grain boundaries.