A series of novel high refractive index episulfide-type optical resins were prepared by ring-opening copolymerization of bis(beta-epithiopropylthioethyl) sulfide (BEPTES) with episulfide derivative of diglydicyl ether of bisphenol A (ESDGEBA) and 2,4-tolylene diisocyanate (TDI), respectively, in the presence of triethylamine as a curing catalyst. The episulfide monomers, BEPTES and ESDGEBA, were synthesized from their corresponding epoxy compounds, respectively. The cured transparent resins exhibit high refractive index (n(d) > 1.63) and relatively low dispersion. The refractive index (n(d)) and Abbe's number (nu(d)) of the BEPTES/ESDGEBA curing system increased linearly with the weight content of BEPTES monomer in the range from 1.633 and 34.0 for the copolymer with 10 wt % of BEPTES to 1.697 and 38.1 for the homopolymer of pure BEPTES. For the BEPTES/TDI curing system, the refractive index and Abbe's number varied linearly with the molar ratio of BEPTES to TDI from 1.652 and 28.7 to 1.669 and 34.6. High glass-transition temperatures (T-g > 130degreesC) of the cured BEPTES/TDI resins were observed, which indicate that the cured BEPTES/TDI resins possess a good heat resistance. The optical, physical, and thermal properties of the episulfide-type cured optical resins were also discussed in this study. (C) 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 2426-2430, 2003.