The anionic terpolymerization in ternary systems comprising oxiranes (ethylene oxide, propylene oxide, cyclohexene oxide, epichlorohydrin), cyclic anhydrides (succinic, phthalic, maleic, glutaric) and SO2 has been studied. It was found that SO2 can act as a propagation catalyst and as a comonomer, which incorporates into the growing chain simultaneously with an oxirane molecule forming alkyl sulfite monomeric units. Such units are susceptible to nucleophilic attack of active centers, which leads to a back-biting and scrambling reaction. It results in the formation of macrocyclic products and gradual SO2 elimination from the polymer chain. On the basis of studies of polymerization products and model reactions, the structures of active species and a scheme of basic elementary reactions have been proposed.