The effect of the macromolecular architecture and composition of block copolymer additives on the reduction of the interfacial tension between two immiscible homopolymers is investigated. A series of (polyisoprene)(2)(polystyrene), I2S, graft copolymers with constant molecular weight and varying composition are utilized as additives in polystyrene/polyisoprene blends. The interfacial tension decreases with the addition of small amounts of copolymer and reaches a plateau at higher copolymer concentration. The interfacial tension at interfacial saturation depends on copolymer composition exhibiting a minimum, which is lower than that using a symmetric diblock with the same molecular weight. Moreover, the interfacial tension at saturation depends on the side of the interface the copolymer is introduced; adding it to the polyisoprene phase is much more efficient than adding it to polystyrene. This is due to the asymmetric architecture of the copolymer and points to the fact that a local equilibrium can only be attained in such systems: the copolymer reaching the interface from one homopolymer phase probably does not diffuse to the other phase. The fact that this behavior is not a kinetic effect is verified using (polystyrene)(2)(polyisoprene), S2I, grafts, which show the mirror image behavior. The effectiveness of the interfacial modifiers is, thus, controlled by the unfavorable interactions, which drive the additive toward the interface, and the possibility of micelle formation, which hinders its activity.