© The Electrochemical Society. It was shown by cyclic voltammetry, current measurements at constant potential and electrochemical impedance spectroscopy that polarization pretreatment of glassy carbon electrodes affects the kinetics of the VO2+/VO2+ and V2+/V3+ redox reactions. After pretreatment at potentials more positive than ~0.7 V (sat. Hg/Hg2SO4), electrodes were less active both for oxidation of VO2+ and for reduction of VO2+. The activity of the electrode remained depressed indefinitely but was recovered by pretreatment at negative potentials. However, the converse was observed for the V2+/V3+ redox reactions: after pretreatment at positive potentials electrodes were more active for oxidation of V2+ and reduction of V3+ than after pretreatment at negative potentials. Both activation and deactivation (for VO2+/VO2+) occurred relatively quickly, typically within ~60 s. and eventually approached a steady state characteristic of the particular pretreatment potential. It is suggested that oxygen species formed on the electrode during anodization inhibit the VO2+/VO2+ electrode reaction but enhance the V2+/V3+ electrode reaction.