This study describes the preparation of polyimide composite membranes by interfacial polymerization/thermal imidization method and their dehydration performance by pervaporation. First, the poly(amic methyl ester) thin layers of the composite membranes were formed on the top of porous polysulfone support by interfacial polymerization of water-soluble diamines -ethylene diamine (EDA), hexamethylene diamine (HXDA) or m-phenylene diamine (m-PDA) - with the organic solvent (toluene)-soluble diacyl chloride-diester - 2,5-bis(methoxycarbonyl) terephthaloyl chloride (BMTC). Next, poly(amic methyl eater) layers of the membranes were converted to polyimide by heating in vacuo. The composite membrane prepared under the proper condition exhibited high separation factor of over 240 and high permeation flux of 1.7 kg m(-2) h for 90/10 (W/W) ethanol/water mixture. The result demonstrated that interfacial polymerization/thermal imidization method could be a very effective method for the preparation of the composite membrane with polyimide ultrathin skin suitable for the dehydration of ethanol/water mixture.