AbstractStereoelective polymerizations performed on substituted oxiranes and thiiranes can yield recovered monomers of high optical purity. The efficiency of the process depends on the chemical nature, the geometry, and the configuration of the chiral ligand of the initiator. It also depends on the nature of the monomer and its enantiomeric composition. When enantiomerically enriched thiiranes were used the stereoelectivity was strongly increased and monomers which were almost optically pure could be isolated using a stepwise procedure. The resolution could be also improved by modifying the initiator with external chiral additives. This stereoelective process seems to be of special interest for the preparation of optically active monomers which are not easily obtained by conventional synthetic methods.