Cationic polymerization of a seven-membered cyclic sulfite (7CS) was carried out with methyl trifluoromethanesulfonate as a catalyst in chlorobenzene. The final conversions of 7CS were 22, 41, 52, and 60% in the polymerizations at 25 degrees C with the initial monomer concentrations of 3, 4, 5, and 6M, respectively. The calculated monomer concentration at equilibrium was evaluated as 2.4M in any case. The conversion of 7CS decreased as the polymerization temperature rose. These results support the fact that this polymerization is an equilibrium one. Delta H-0 and Delta S-0 in the polymerization were evaluated as -0.765 kcal/mol and -4.18 cal/mol by Dainton’s equation, respectively.