The interfacial tension (γ) in ternary blends of unsaturated polyester (UP) and rubber oligomers in styrene have been measured using three different techniques: tensiometric plate, capillary height, and drop profile methods. These experimental methods give similar results, but it has been shown that γ values “with or without skimming” are very different. The presence of small molecules, inhibitors, impurities, etc., in the interphase region can explain this discrepancy. We have observed a good correlation between γ and the Flory-Huggins interaction parameter. The elastomer chain ends have an influence on the interfacial tension: γ is reduced when the oligomers possess hydroxyl groups. An epoxy terminated butadiene–acrylonitrile copolymer, ETBN, was prepared from a large excess of diglycidyl ether of bisphenol A, DGEBA, and carboxyl terminated butadiene–acrylonitrile copolymer, CTBN, at 150°C, and without a catalyst. The excess of DGEBA was eliminated by successive precipitation in the selective solvent ethanol. Since the DGEBA blocks are miscible with the UP resin, the introduction of the ETBN copolymer drastically reduces the interfacial tension of the ternary blend of UP and rubber in styrene.