In this paper we report on the electrochemical performance of the symmetric carbon/carbon electrochemical capacitor with water-based electrolyte, operating at the temperatures down to -40 degrees C. The electrolyte solution consists of inorganic salt based on the NO3- anion and Mg2+ as a divalent cation, dissolved in the mixture of water and organic solvent. The electrochemical performance has been verified at slightly elevated (up to +40 degrees C) and low temperatures (down to -40 degrees C) by cyclic voltammetry, galvanostatic charge/discharge and electrochemical impedance spectroscopy techniques. In the liquid electrolyte range (down to -23 degrees C) the capacitance values did not differ remarkably along the temperature (ca. 100 F g(-1) per electrode); at the temperature of -40 degrees C (i.e. below the freezing point), the capacitance value was still retained at more than 50% of initial value. Moreover, stable performance of the capacitor at +40 degrees C and the voltage of 1.4 V has been proven by 60 h of floating test and galvanostatic cycling with no remarkable performance fade (10% of capacitance drop after 15 000 galvanostatic cycles at 1 A g(-1) current load).