Different types of graphite-like carbons, namely Sibunits, are studied as catalysts in the liquid-phase oxidation of sodium sulfide of high concentrations (28-59 g/l) into sodium sulfate by oxygen, at the temperature range of 90 to 130^oC and oxygen pressure of 10 atm. A novel electrochemical mechanism for this liquid phase oxidation by molecular oxygen, which explains most of the experimental data, is suggested.