The limiting reduction and oxidation potentials and electrolytic conductivities of new quaternary ammonium salts were examined for electrochemical capacitor applications, whose anions have already been tested as lithium salts for lithium battery applications. The anodic stability was in the following order BR4- < ClO4- less than or equal to CF3SO3- < (CF3SO2)(2)N- less than or equal to C4F9SO3- < BF4- < PF6- less than or equal to AsF6- < SbF6- The electrolytic conductivities of Me4-nEtnN(CF3SO2)(2)N (n = 0 - 4) were examined in comparison with Me4-nEtnNBF4 counterparts. These imide salts showed good solubility, relatively high conductivity, and anodic stability in propylene carbonate. Et4N(CF3SO2)(2)N was found to be a good supporting salt for low permittivity organic solvents, and it afforded a highly conductive electrolyte system based on the ethylene carbonate-dimethyl carbonate mixed solvent, which is useful for electrochemical capacitor applications.