The syntheses and cationic polymerizations of seven-membered cyclic sulfites having phenyl (7PhCS), benzo (7BnCS), and cyclohexane (7CyCS) groups were examined. 7PhCS and 7CyCS underwent complete elimination of SO2 to afford the corresponding tetrahydrofuran derivatives under the cationic conditions. On the other hand, 7BnCS underwent cationic polymerization in the presence of cationic initiators such as TfOH, TfOMe, SnCl4, TsOMe, and MeI in bulk or chlorobenzene to afford the corresponding polymer with (M) over bar(n) 950-8700 without desulfoxylation. The benzo group was operative for polymerization and suppression of desulfoxylation. The polymerization of 7BnCS did not show equilibrium character. The steric hindrance of the benzene ring might be effective to suppress a back-biting reaction. The volume expansion during the polymerization of 7BnCS was 8.57%. This might be caused by the larger dipole-dipole interactions between the monomers and smaller ones between the polymers.