Abstract: A process for preparing diacetylbenzene by a liquid phase oxidation reaction which comprises contacting a heterogeneous mixture of (A) about 98 to about 25 percent by volume of an organic liquid phase containing at least one compound selected from the group consisting of diethylbenzene and ethylacetophenone and (B) about 2 to about 75 percent by volume of an aqueous liquid phase containing, as a catalyst, at least about 0.01 mol/l of at least one water-soluble salt of a metal selected from the group consisting of manganese, cobalt, nickel, chromium, copper and iron and having a pH of about 1 to about 6 with a molecular oxygen-containing gas at a temperature of from about 100.degree. C. to about 180.degree. C. under a pressure of about 1 to about 50 Kg/cm.sup.2 to effect the liquid phase oxidation reaction, separating the resulting reaction mixture liquid into the aqueous liquid phase and the organic liquid phase and recovering diacetylbenzene from the separated organic liquid phase by distillation.