Copolycondensation of isophthaloyl chloride, bisphenol A, and aliphatic diols with additional reactive groups were performed in the presence of triethylamine by a trimethyl phosphate/ cyclohexane organic/organic interfacial method. The composition of diol and bisphenol A residues in the resulting copolyesters is very close to that in the feed from the initial stage of reaction. The resulting copolyesters with reactive groups can be used for the preparation of various functional polymers. The mechanism of an organic/organic interfacial polycondensation was also discussed.