The hydrous form of ruthenium oxide (RuO[sub 2][center dot]xH[sub 2]O) has been demonstrated to be an excellent electrode material for electrochemical capacitors. This material, as prepared by a sol-gel process at low temperatures, is amorphous and electrically conductive. The specific capacitance is over 720 F/g. This value is at least two times higher than the highest value ever reported for such materials. The charge storage mechanism is believed to involve bulk electrochemical protonation of the oxide. This discovery opens a new avenue of research in the field of high energy density electrochemical capacitors.