The functional composition and electrochemical behavior of the samples of N121 oxidized nanosized technical carbon in aqueous electrolytes are studied. For oxidation a 30% aqueous hydrogen peroxide (H2O2) solution and 2% H2O2 with the addition of singlet oxygen or ozone were used. By means of X-ray photoelectron spectroscopy data and the analysis of the near edge X-ray absorption fine structure the features of the chemical structure of the samples are found. The oxygen concentration did not exceed 5 at.% in the samples. The analysis of cyclic voltammograms reveals that at low scan rates the specific capacitance of the material is determined by the functional composition of the surface. The sample oxidized by 30% H2O2 solution and containing the largest number of–OH and–COOH groups demonstrated the highest capacitance in 6M KOH and in 1М H2SO4 it was the sample with the highest concentration of C=O groups formed during the oxidation with singlet oxygen. The stability of carbon electrodes is studied in supercapacitor models.