A catalyst system of ZnBr2/n-BU4NI is reported for the first time as a highly efficient binary catalyst for the synthesis of styrene carbonate (SC) from styrene oxide and carbon dioxide under supercritical conditions. In a very short reaction time (30 min) and even at a mild reaction temperature (80 degrees C), 100% selectivity and nearly 100% yield of SC have been obtained; the TOF value obtained is 46 times larger than those so far reported under similar conditions. The effects of the ZnBr2/n-Bu4NI ratio, types of cation and anion, and other reaction conditions on the reaction were investigated. On the basis of the results, a reaction mechanism has been proposed. (c) 2005 Elsevier B.V. All rights reserved.