A mathematical model of an electrochemical capacitor with hydrous ruthenium oxide (RuO2. xH(2)O) electrodes including both double-layer and surface faradaic processes is developed to predict the behavior of the capacitor under conditions of galvanostatic charge aid discharge. The effect of RuO2. xH(2)O particle size is studied and shows that dir smaller the particles the better the performance because of the increased surface area per unit volume or mass. The model also predicts that the faradaic process increases significantly the energy per unit volume of the capacitor for power densities of 100 kW/L or less.