The reactions of SO2 with various oxiranes, propylene sulfide, and oxetane have been studied. It was found that SO2 activates these monomers toward ring opening polymerization. The spontaneous reactions lead to the formation of cationic active centers, on which the heterocyclic monomer homopropagation and various cyclization processes mainly proceed. Anionic alternating copolymerization of SO2 with ethylene oxide or propylene oxide was observed after the spontaneous termination of cationic active species. Zwitterions with active anionic sites are formed in the presence of pyridine. They can initiate the alternating copolymerization of SO2 and oxiranes. The relative reactivity of the oxiranes in the addition to anionic species increases in the following order : epichlorohydrin < glycidyl acetate < propylene oxide butyl glycidyl ether < ethylene oxide < 1,2-cyclohexene oxide. Styrene oxide, oxetane, and propylene sulfide undergo spontaneous reactions also in the presence of pyridine. In systems with the former two monomers the alternating copolymerization proceeds simultaneously with the processes initiated by cationic centers; propylene sulfide, however, does not form copolymers with SO2. Monomers of low ring strain like THF or trioxane do not undergo ring opening in the presence of SO2 and pyridine.