A hybrid electrochemical capacitor using MnO2 and Fe3O4 as active material for the positive and the negative electrode, respectively, has been designed. The electrodes have been individually tested in a mild aqueous electrolyte (0.1 M K2SO4) to define the adequate balance of active material in the capacitor as well as the working voltage of a capacitor based on these two electrodes. The specific capacitances of MnO2 and Fe3O4 were 150 +/- 10 and 75 +/- 8 F/g, respectively whereas the specific capacitance of the Fe3O4/MnO2 capacitor was equal to about 20 F/g of active material. The hybrid electrochemical capacitor has been cycled between 0 and 1.8 V for over 5000 constant current charge/discharge cycles. A real energy density of 7 Wh/kg was reproducibly measured with a real power density up to 820 W/kg. (C) 2003 The Electrochemical Society.