Concentrated NaClO4 aqueous solutions have been proposed as the electrolytes in electric double-layer capacitors. The advantages of this kind of electrolytes have been addressed in the terms of enlarged specific capacitance, enhanced rate capability and elevated low-temperature performance of porous carbon electrodes. Thermal analysis, ionic conductivity measurement and Raman spectroscopic investigations have been performed on the NaClO4 aqueous solutions in conjunction with the electrochemical study of porous carbon electrodes in this kind of electrolytes. The correlation between the hydration number of ions in the solutions and capacitive behavior of porous carbon has been clarified. The rate performance improvement in porous carbon electrode has also been connected to the increase in ionic conductivity of the electrolytes. The enhanced capacitance retention of porous carbon electrode at low temperatures in concentrated solutions has been ascribed to a fall of freezing point.