It is shown by cyclic voltammetry and current measurements at constant potential that, when glassy carbon electrodes are pretreated by polarization at potentials more positive than 0.5 V (sat. Hg/Hg2SO4), they are subsequently less active for oxidation of VO2+. The activity of the electrode is recovered by polarization at negative potentials. At any given potential a characteristic amount of electrode deactivation occurs in a relatively short time (∼60 s) and further pretreatment at that potential does not further deactivate the electrode. It is suggested that the level of deactivation is determined by processes that are primarily dependent on potential rather than on kinetics and involve the formation of oxygen species that interact with active sites on the carbon surface.