The promotional effect Of CO2 in the liquid phase oxidation of ethylbenzene to acetophenone (AP) with the Co/Mn/Br (MC-type) catalyst system has been investigated. Significant enhancement of catalytic activity was found in the presence Of CO2 and thus selectivity toward acetophenone was increased as well. UV-visible investigations of the MC-type catalyst system in the oxidation of ethylbenzene reveal that Mn(HI) is more easily formed in the presence Of CO2; such formation would be associated with the CO2 promotional effect.