New polythioesters have been obtained by polycondensation of 4,5di(mercaptomethyl)-l,2dimethylbenzene, 4,6di(mercaptomethyl)-l,3dimethylbenzene, and 2,5di(mercaptomethyl)-1,4dimethylbenzene with isophthaloyl and terephthaloyl chlorides. To define the optimal conditions of polycondensation, polythioester from 2,5di(mercaptomethyl~l,4dimethylbenzene and isopthaloyl chloride was choosen as a model system. The optimalization of the polycondensation conditions was carried out by low-temperature and high-temperature S(F lution polycondensation as well as by interfacial polycondensation. To determine the optimal conditions for polycondensation the following parameters have been studied. The kind of organic solvent, concentration of reagents, temperature and time of reaction, and, in the case of interfacial polycondensation additional, the ratio of aqueous to organic phase, kind and concentration of hydrochloride acceptor, kind and concentration of catalyst and concentration of emulsifier (Mersolan) were taken into consideration. The synthesis of all described polythioesters obtained by polycondensation di(mercaptomethylMimethy1benzenes with isophthaloyl and terephthaloyl chlorides was carried out under the same optimal conditions, established for the reaction 2,5di(mercaptomethyl~l,4dimethylbenzene with isophthaloyl chloride. The structure of polythioesters was determined from infrared, X-ray, and elementary analysis. Some physicochemical, thermal, mechanical, and electrical properties as well as molecular weight of polythioesters have been determined.