Cationic ring-opening polymerization behavior of a seven-membered cyclic sulfite (1)was examined. 1 was prepared by the reaction of 1,4-butanediol with SOCl2 in 58% yield. The cationic polymerization of 1 was carried out at 0, 25, 60, or 100 degrees C with trifluoromethanesulfonic acid (TfOH), methyl trifluoromethanesulfonate (TfOMe), BF3 . OEt2, SnCl4, methyl p-toluenesulfonate (TsOMe), or MeI as an initiator in bulk under a nitrogen atmosphere to afford the polymer with (M) over bar(n) 1000-10,400. The order of activities of the initiators for 1 was as follows, TfOH congruent to TfOMe > SnCl4 > BF3 . OEt2 > TsOMe congruent to MeI. The polymerization of 1 with TfOMe afforded a poly(sulfite) below 25 degrees C, but afforded a polymer containing an ether unit at 60 degrees C, which was formed by a desulfoxylation. The higher the activity of the initiator was, the more easily the desulfoxylation occurred. We expected volume expansion on polymerization because cyclic sulfites have large dipole moment values, but it turned out that 1 showed 4.34% shrinkage on polymerization.