Interfacial agents used in the compatibilization of immiscible polymer blends often consist of block copolymers containing at least one segment compatible with each of the two phases of the blend. This work examines the influence of the molecular weight, architecture, and chemical composition of the interfacial agent on its ability to emulsify a polymer blend. The system chosen is a blend containing 80% polystyrene and 20% ethylene-propylene rubber, compatibilized by diblock copolymers of poly(styrene-hydrogenated butadiene). The emulsification curve, which relates the dispersed phase particle size to the concentration of interfacial agent added to the system, was used as a tool to characterize the efficacy of the different interfacial agents. The observed behavior is similar to that of classical emulsions : a rapid drop in phase size at low concentrations of interfacial modifier, followed by a levelling off to an equilibrium diameter value once a "critical" concentration has been reached. For systems compatibilized by symmetrical diblocks (i.e., containing approximately 50% styrene by weight), the volume average particle diameter decreased from 2.7 mu m for the unmodified system to about 0.4 mu m once interfacial saturation is reached. The critical concentration for emulsification decreased with increasing interfacial agent molecular weight, due to the higher interfacial area occupied by longer molecules; however, this parameter did not affect the equilibrium particle diameter. The asymmetrical diblock copolymer (30% styrene) was found to be less effective than the symmetrical ones over the entire range of concentrations studied (5 to 35% modifier, based on the volume of the minor phase). Asymmetrical diblock copolymers would tend to form micelles, whereas symmetrical copolymers are less constrained at the interface. No significant difference was observed between the emulsifying capability of tapered and pure diblocks of similar composition and molecular weight.