An emulsification curve which tracks the change in dispersed phase size with interfacial modifier concentration and a fracture mechanics approach have been used to study the behavior of a variety of well-defined interfacial modifiers for a melt-processed ethylene-propylene rubber (EPR)/polystyrene (PS) blend. In this study two poly(styrene/ethylene-butylene) (SEB) diblock copolymers and a poly(styrene/ethylene-propylene) (SEP) diblock copolymer were compared. All of the above interfacial modifiers demonstrate an excellent emulsification efficacy for the EPR/PS system, and no effect was observed when changing the block structure from ethylene-butylene to ethylene-propylene. Despite the excellent capacity of all three diblock modifiers to emulsify the blend, the Charpy and notched and unnotched Izod impact testing demonstrate significantly different behaviors for the various copolymers, The higher molecular weight SEB (M-n = 187 000) and SEP (M-n = 140 000) interfacial modifiers displayed a brittle fracture behavior over a wide range of interfacial modifier concentrations. The lower molecular weight SEE diblock (M-n = 67 000) displays a brittle to ductile transition in impact closely related to the concentration of modifier required for saturation of the interface. Its ability to improve the properties is due to a high areal density of polymer chains at the interface compared to the two higher molecular weight diblock copolymers as directly estimated from the emulsification curve. These results underline the difficulty of achieving high areal densities of copolymer at the interface in melt-processed systems. In addition, the mechanical performance of this effective lower molecular diblock was compared with a triblock, poly(styrene/ethylene-butylene/styrene (SEBS), of similar molecular weight. The diblock and triblock copolymers both demonstrate the ability to substantially improve the mechanical propel ties of PS/EPR once interfacial saturation is achieved and possess a virtually identical behavior despite the fact that the styrene block in the triblock copolymer is below its entanglement molecular weight, The similar performance of the triblock and diblock copolymers appears to be due in large part to a similar areal density of joints across the interface.