Electrochemical behavior of electrodes based on activated carbon is studied by recording voltammetric curves and using the impedance method. A quantitative relation between hydrophobic and hydrophilic pores is established. Hydrophilization of activated carbon is performed. The occurrence of a reversible surface reaction is discovered in the region of negative potentials, where it was not observed earlier. A possible mechanism for the reaction is proposed and its kinetic parameters are found. The existence of surface compounds stable only in electrolyte (H2SO4) is established. A dramatic drop of the EDL capacitance at negative potentials is discovered.