The oxidation of ethylbenzene with 30% aqueous hydrogen peroxide (H2O2) in an acetone-water medium at 30 degrees C was investigated using hexadentate 8-quinolinolato manganese( Ill) complexes (Q(3)Mn(III)) as catalysts. The results indicated that the Q(3)Mn(III) complexes, with ammonium acetate and acetic acid as additives, selectively catalyzed the side chain oxidation of ethylbenzene at secondary carbon atoms, affording acetophenone as a major product. Among the Q(3)Mn(III) complexes examined, the 5,7-dibrominated Q(3)Mn(III) catalyst was the most active, and provided the ethylbenzene conversion of 26.1% and the oxidation yield of 65% under the optimum conditions. The influence of various parameters on the reaction was checked in detail. Based on the UV-vis spectra, a free radical mechanism for the Q(3)Mn(III) catalytic system was also proposed. (C) 2010 Elsevier B.V. All rights reserved.