The effect of several parameters such as molecular weight, chemical composition and architecture of the interfacial agent on the dispersed particle size is investigated in polystyrene/ethylene-propylene rubber blends. The study is based on the dependence of the dispersed phase size versus the concentration of interfacial modifier, the so-called emulsification curves. A critical concentration value of interfacial agent based on the volume of the minor phase was determined beyond which no further changes in particle size were observed. These results show that block copolymers also display an emulsification curve. The influence of the molecular weight (from 50 x 10(3) to 174 x 10(3) g mol(-1)) of the saturated triblock poly(styrene-block-ethylene-butylene-block-styrene) interfacial modifier is demonstrated to have little effect on emulsification phenomena. The influence of the chemical composition of the interfacial modifier on the shape of the emulsification curve appears to be a critical consideration. The presence of double bonds in the copolymer modifier significantly diminishes its emulsification capability. It is interesting to note that, once the emulsification phenomenon has been significantly reduced due to the presence of double bonds, imposing a conformational restriction (star-shape architecture) has little effect on the emulsification curve.