A multiphase system consisting of a hydrocarbon solvent, a strong alkaline solution, and a quaternary onium salt, in the presence of a Pd/C catalyst with hydrogen at atmospheric pressure, allows the rapid and progressive displacement of the chlorine atoms from polyhalogenated benzenes. The onium salt in this case constitutes a third liquid phase in which the reaction takes place. At 50" C, 1, 2, 4, 5-tetrachlorobenzene is reduced to ...