Oxygen-terminated diamond electrodes were prepared by exposing as-grown hydrogen-terminated diamond thin films to oxygen plasma. The as-grown surfaces, which were highly hydrophobic, become hydrophilic after the oxygen plasma treatment. The apparent surface conductivity was not significantly changed after the oxygen plasma treatment. However, the electrochemical responses to several redox systems became remarkably different. For example, the cyclic voltammetric anodic-cathodic peak separations for the oxygen-terminated diamond electrodes became extremely large compared to those for the as-grown electrodes. This behavior was examined in comparison with as-grown diamond and glassy carbon electrodes.