Electrochemical capacitors made with hydrous ruthenium oxide (RuO2;xH(2)O) and hydrogen inserted hydrous ruthenium oxide (H6RuO2.xH(2)O) electrode materials were investigated. The electrochemical behavior of RuO2.xH(2)O in the potential range of -0.2 to 1.3 V vs. saturated calomel electrode (SCE) was characterized using a cyclic voltammetry technique. It was found that, electrochemically, amorphous RuO2.xH(2)O electrodes behave quite differently from crystalline RuO2. The potentials of both the positive and the negative electrodes of the capacitors were monitored relative to SCE. For capacitors made with RuO2.xH(2)O electrode material, gas evolution and an increase of cell resistance were observed in the first few charge/discharge cycles. This was due to the fact that the potentials of the starting materials were close to the oxidation limit of the potential window. Hydrogen inserted hydrous ruthenium oxide with potentials near the center of the potential window were prepared using both electrochemical and chemical methods. It was demonstrated that capacitors made of hydrogen inserted ruthenium oxide electrodes exhibited high efficiency, high energy, and high power through a cycle life over 60,000 times.