New stricter regulations are being imposed on diesel quality for sulfur content worldwide. Recalcitrant aromatic sulfur compounds such as 4,6-dimethyldibenzothiophene (4,6-DMDBT) are resistant to removal by hydrodesulfurization, because of their low reactivity and sterically hindering structure. Oxidative desulfurization is one of the alternatives that are being investigated for this purpose. In this study, an oxidative system composed of sodium carbonate and hydrogen peroxide was investigated in a biphasic system of diesel and acetonitrile. Ultrasonic methods were applied for the intensive mixing of the biphasic system. The effects of different operating conditions-including temperature, amount of oxidant, and the base concentration-were investigated. Reduction of the DMDBT content by >90% was achieved in the diesel samples. The activation energy of pseudo-first-order reaction for DMDBT was determined to be 57 kJ/mol.