The influence of water on the polymerization of thiirane and 2-methylthiirane with CdCO, was studied. The polymerization process takes place only when a large interface monomerwater is formed. This is achieved by the presence of an amount of water in the reaction mixture which is comparable with that of the monomer. The polarity of the solvent does not influence the rate of formation of poly(2-methylthiirane). The polymerization is carried out in the mass of monomer or in the presence of an alcohol and a solvent not miscible with water.