One of the key factors to a better understanding of the phase morphology of high-impact ethylene-propylene (EP) copolymers is the knowledge of the rheological behaviour of the various phases in these complex systems. Next to the molecular weight distribution, the comonomer content also affects the viscosity, as could be demonstrated in a systematic study of a wide range of compositions. More specifically, this means that for a given average molecular weight, the zero shear viscosity of the disperse phase consisting of amorphous ethylene-propylene 'rubber' (EPR) and crystalline polyethylene (PE) of such an impact copolymer are up to orders of magnitude higher than the respective polypropylene (PP) phase. This has a significant effect on the phase morphology and ultimately the mechanical performance of these compositions.