A hybrid electrochemical capacitor using MnO2 and activated carbon (AC) as positive and negative electrodes, respectively, has been designed. The electrodes were individually tested in a mild aqueous electrolyte (0.65 M K2SO4) in order to define the adequate balance of active material in the capacitor as well as the working voltage. The hybrid electrochemical capacitor was cycled between 0 and 2.2 V for over 10,000 constant current charge/discharge cycles. A real energy density of 10 Wh/kg was reproducibly measured with a real power density reaching 3600 W/kg. The hydrogen and oxygen evolution reactions on AC and MnO2 electrodes, respectively, were investigated in 0.65 M K2SO4. Despite the good electrochemical performance of the 2.2 V capacitor, gas evolution could be a hindrance for practical use. Subsequently, a 1.5 V capacitor was tested for more than 23,000 cycles and yielded interesting electrochemical performance with negligible gas evolution. (C) 2004 The Electrochemical Society.