The mechanism of the vanadium VO2+/VO2+ redox couple has been examined in acidic aqueous solutions. A detailed understanding of this chemistry is of interest for improving and optimizing the performance of vanadium redox-flow batteries, a promising electrochemical electricity storage technology. The vanadium 4+/5+ redox reactions were studied at a rotating disk graphite electrode and polarization curves were obtained in sulfuric acid and perchloric acid, with varying pH and vanadium concentrations. The results were compared to model predictions for different mechanisms. The data were consistent with a model with a multistep chemical-electrochemical-chemical mechanism at low overpotentials, which changes to a multistep electrochemical-chemical-chemical mechanism at higher anodic or cathodic overpotentials. Unusually high Tafel slopes (350-450 mV/decade) were observed for the reduction of VO2+ at higher overpotentials. While this could not be directly explained by the model, insights gained through the use of the model can provide the basis for some suggestions. (C) 2003 The Electrochemical Society.