Ethylene-propylene-diene-terpolymer (EPDM) and polypropylene (PP)-based uncross-linked and dynamically cross-linked blends were prepared both in an internal mixer and in a corotating twin-screw extruder. The effects of composition, plasticization and mixing equipment on the morphology development and the final viscoelastic properties were studied. In the uncross-linked blends, the plasticization resulted in a coarser morphology. Furthermore, it was shown that the majority of the plasticizer resided in the EPDM phase, enabling its deformation in the flow direction. In addition, the intensive mixing conditions inside the twin-screw extruder resulted in a finer morphology. In the dynamically cross-linked blends, the twin-screw extrusion process resulted in a higher level of gel content with larger EPDM domains. The plasticization showed again a coarsening effect, resulting in interconnected cross-linked EPDM domains. An interesting interfacial phenomenon was observed especially in the plasticized vulcanized blends where nanometer size occluded PP domains were stripped off and eroded into the EPDM phase. With the exception of the nonplasticized uncross-linked blends, the viscoelastic properties of all other blending systems were found to be directly affected by the morphology, gel content (in the case of cross-linked blends), and the presence of the plasticizer. POLYM. ENG. SCI., 2012. (C) 2011 Society of Plastics Engineers