The effects of the molecular structure of the styrene-isoprene block copolymer on the interfacial tension, the morphology and the interfacial adhesion of polystyrene/polyisoprene were investigated. A reduction in interfacial tension is observed with the addition of a small amount of copolymer, followed by a leveling off as the copolymer concentration exceeds the critical micelle concentration. The reduction in interfacial tension between polystyrene and polyisoprene is more significant when the isoprene-rich diblock copolymer is added than the cases when the symmetric or styrene-rich diblock copolymer is added. The interfacial tension data seem to be consistent with the phase morphology and the interfacial adhesion : the lower the interfacial tension, the smaller the domain size of dispersed phase and the better the interfacial adhesion.