Sulfur dioxide undergoes copolymerization with ethylene oxide in the presence of Lewis bases capable of forming onium ions (such as amines, phosphines, diethyl sulfide, dimethyl sulfoxide, N,N-dimethylformamide) yielding poly(ethylene sulfite) bearing also a small amount of ethylene oxide monomeric units homosequencies. After 4–6 h of reaction at 50°C in bulk, the monomer conversion reaches 70–90%. The copolymer obtained is a mixture of linear and cyclic products which tend to degrade to yield ethylene sulfite. At 50°C this decomposition is very slow when unreacted SO2 is completely removed. However, it proceeds very rapidly, within a few hours, in the presence of compounds with strong electrophilic properties like BF3, H3PO4, or chlorotrimethylsilane. Studies of model reactions indicate that in the systems studied the macrocyclization results mainly from the attack of sulfite or alcoholate active species at the carbon atoms in polysulfites. Chain transfer reactions with the solvent occur also in reactions carried out in ethanol.