Anionic ring-opening polymerization of epsilon-thionocaprolactone was examined. When organolithiums, Grignard reagents, and Lithium tert-butoxide were used as the initiators, the corresponding polythiocarboxylic-O-ester was selectively formed. When potassium tert-butoxide and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) were used, the corresponding polythiocarboxylic-S-ester was predominantly formed. The monomer conversion and M-n of the obtained polymer increased with the polymerization temperature. Elongation of polymerization time resulted in decrease of the polymer yield and M-n. The polymerization in tetrahydrofuran (THF) proceeded faster compared with the polymerization in toluene. Formation of a cyclic dimer was observed in the polymerization in THF.