Dibenzothiophenes are oxidized effectively with H2O2 in the presence of acid catalysts in a tetradecane/AcOH biphasic system. The oxidation proceeds in the AcOH phase and most of the oxidation products stay there, resulting in the successive removal of the dibenzothiophenes from the tetradecane phase. Using this biphasic oxidation system can effectively reduce the sulfur content in diesel oil.