The mechanism of deterioration of the capacitive response of electrodeposited MnO2 under reducing potentials has been studied by combined electrochemical and quartz crystal microbalance, XPS and SEM techniques. Cycling of the electrode towards cathodic potentials of −0.3 V vs. Ag/AgCl leads to irreversible reduction resulting in low valence manganese oxide, most probably Mn2O3. The reduction is accompanied by phase transformation with the uptake of considerable amounts of water into the layer. The capacitive behaviour of the MnO2 electrode fades with prolonged cycling with the concomitant change of charging/discharging mechanism.