Blends of polystyrene and ethylene-propylene rubber are compatibilized by different block copolymer interfacial modifiers using melt processing. The emulsification curve, which relates the minor phase particle average diameter to the concentration of emulsifier, is used to compare the efficacy of these modifiers for the interface. Blends containing 90% PS/10% EPR and 80% PS/20% EPR were modified by diblock and triblock copolymers of equal molecular weight and chemical composition. At 10% minor phase, the triblock copolymer acts as a more efficient emulsifier than the diblock, as evidenced by the lower equilibrium diameter (0.35 mu m for the triblock, as opposed to 0.6 mu m for the diblock) and the lower critical concentration for emulsification. At 20% minor phase, the emulsification curve demonstrates a shift in both critical concentration for emulsification and equilibrium particle diameter, which suggests strong evidence of micellar formation for the triblock copolymer. Even at concentrations lower than the critical concentration for emulsification (C-crit), micelle formation is suggested by the low values of interfacial area occupied per molecule for the triblock copolymer in the 80:20 blends, although a transmission electron microscopic study performed on a similar system did not detect any micelle formation at subcritical concentrations. The blends compatibilized by the diblock copolymer, however, show the same equilibrium diameter and critical concentration for emulsification for both blend compositions, indicating that even at 20% minor phase, all the diblock modifier migrates to the interface.