A hydrous ruthenic acid (H0.2RuO2.1 . nH(2)O) possessing a layered structure with a crystalline framework was prepared by proton exchange of a layered potassium ruthenate. Its electrochemical capacitor behavior was studied by cyclic voltammetry in various electrolytes. Specific capacitance up to 390 F g((RuO2))(-1) at a scan rate of 50 mV s(-1), which is a ten-fold increase compared to conventional anhydrous RuO2, was obtained using layered ruthenic acid hydrate in 0.5 M H2SO4 electrolyte. Slightly lower values were obtained for 1 M HClO4 and 1 M KOH electrolytes. The difference in the specific capacitance of anhydrous ruthenium oxide, hydrous ruthenium oxide, hydrous ruthenic acid, and restacked ruthenic acid nanosheets in the various electrolytes is discussed. (C) 2004 The Electrochemical Society.