The modification of the interface in immiscible polymer blends is critical in order to optimize physical properties. Despite the great commercial importance of polymer blends, many aspects of the emulsification process remain unclear. In this paper, it will be shown that an emulsification curve can be used to estimate the influence of the architecture and molar mass of styrene/hydrogenated butadiene block copolymer interfacial modifiers on the emulsification of polystyrene/ethylene-propylene rubber blends under melt processing conditions. An expression for estimating the critical amount of copolymer to saturate the interface, as well as aspects concerning the localisation of the modifier in the blend system will also be addressed.