The effects of diblock copolymer addition on the interfacial tension of immiscible homopolymer blends are examined for the ternary system comprising polystyrene (PS), poly(dimethylsiloxane) (PDMS), and poly(styrene-b-dimethylsiloxane) [P(S-b-DMS)]. Interfacial tension is measured, as a function of the diblock copolymer concentration and the molecular weight of PDMS, using an automated pendant drop tensiometer. The interfacial tension of the blend initially decreases upon an increase in the copolymer concentration and then attains a constant value above a certain critical concentration. A maximum interfacial tension reduction of 82% is achieved at a critical concentration of 0.002% diblock copolymer. At a fixed PS molecular weight, the reduction in interfacial tension increases and the critical concentration decreases with an increase in the PDMS molecular weight. These results are compared to the predictions of Leibler’s theory for copolymer brushes wherein the critical concentration can be attributed either to saturation of the interface (i.e., for the dry brush case) or to the onset of copolymer micelle formation (i.e., for the wet brush case). The degree of interfacial tension reduction is found to be dependent on the sample preparation procedure. When the copolymer is mixed into the PS phase, the amount of interfacial tension reduction is much less than the reduction when it is blended into the PDMS phase. This behavior suggests that the polymer blend interface may act as a kinetic trap that limits the attainment of global equilibrium in these systems.