Poly(ethylene-co-propylene-co-diene) (EPDM) containing 50 wt% of poly(ethylene-co-propylene) grafted with maleic anhydride (EP-g-MA) has been melt blended with poly(styrene-co-acrylonitrile) (SAN) added with various amounts of reactive SAN, i.e. SAN bearing either primary amine (SAN-NH2) or carbamate groups (SAN-carb). Carbamate groups are precursors of primary amines by thermal thermolysis during melt processing. These reactive systems are good models for studying the effect of the kinetics of the interfacial reaction on the phase morphology. Reaction of maleic anhydride with the primary amine is indeed very fast, at high temperature, in contrast to the reaction with carbamate, which is controlled by the carbamate thermolyis into primary amine. Special attention has been paid to the experimental conditions required for the development of the particle-in-particle morphology for the dispersed phase. Depending on the mixing sequence and the grafting kinetics (NH2/MA versus carb/MA), this particular phase morphology can be forced or occurs spontaneously.