This work examines the influence of well-defined diblock copolymers on the interfacial tension and morphology of an immiscible polymer blend. The system studied is a blend of 80% polystyrene and 20% ethylene propylene copolymer compatibilized by diblock copolymers of poly(styrene-hydrogenated butadiene). The diblocks differed in that one contained 53% styrene (symmetric diblock) and the other 30% styrene(asymmetric diblock). The interfacial tension was performed using the breaking-thread technique, and the results were compared to both the theoretical predictions of Noolandi and Hong, and Leibler. The morphology was tracked using an emulsification curve, comparison of which with the interfacial tension results made it possible to estimate the relative role of interfacial tension adn coalescence reduction in particle-size reduction phenomena.