Dithiolane-2,4-dione was polymerized either by heating in bulk to 140 degrees C or by reacting at 20 degrees C using pyridine and triethylamine as catalysts. Whereas the thermal polymerization almost exclusively yielded cyclic poly(thioglycolide), the samples prepared at 20 degrees C by means of tertiary amines contained cyclic and linear chains. The end groups of the linear chains varied with the nonsolvent used for the workup procedure, because chains having reactive end groups were immobilized by rapid crystallization and reacted at the electrophilic CO-chain end with the nonsolvent. D,L-5-Methyldithiolane-2,4-dione proved so stable that tertiary amines did not catalyze its polymerization even at 100 degrees C. Thermal polymerization at 160 degrees C yielded oligoester mainly consisting of cyles. A zwitterionic polymerization mechanism allowing for simultaneous chain-growth and step-growth polymerization is discussed.