Cationic ring-opening polymerization of epsilon -thionocaprolactone was examined. The corresponding polythioester with the number-average molecular weight (M-n) of 57,000 was obtained in the polymerization with 1 mol % of BF3 . OEt2 as an initiator in CH2Cl2 at 28 degreesC for 5 h with quantitative monomer conversion. The M-n of the polymer increased with the solvent polarity and monomer-to-initiator ratio. No polymerization took place below -30 degreesC, and the monomer conversion and M-n of the polymer increased with the temperature in the range of -15 to 28 degreesC. The increase of initial monomer concentration was effective to improve the monomer conversion and the M-n of the obtained polymer.