Melt rheological behavior, phase morphology, and impact properties of isotactic-polypropylene (iPP)-based blends containing ethylene-propylene copolymer (EPR) synthesized by means of a titanium-based catalyst with very high stereospecific activity (EPR Ti ) were compared to those of iPP/EPR blends containing EPR copolymers synthesized by using a traditional vanadium-based catalyst (EPR V ). The samples of EPR copolymers were synthesized ad hoc. They were characterized by comparable propylene content, average molecular masses, and molecular mass distribution in order to assess the effects of distribution of composition and sequence lengths of the structural units on the structure-properties correlations established in the melt and in the solid state while studying different iPP/EPR pairs. 1-5 Differential scanning calorimetry, (DSC), wide-angle X-ray spectroscopy (WAXS), small-angle X-ray (SAXS), and scanning electron microscopy (SEM) investigations showed that the EPR Ti chain is characterized by the presence of long ethylenic sequences with constitutional and configurational regularity required for crystallization of the polyethylene (PE) phase occurring, whereas a microstructure typical of a random ethylene-propylene copolymer was exhibited by the EPR V copolymer. The different intra-and intermolecular homogeneity shown by such EPR phases was found to affect their melt rheological behavior at the temperatures of 200 and 250°C; all the EPR Ti dynamic-viscoelastic properties resulting were lower than that shown by the EPR V copolymer. As far as the melt rheological behavior of the iPP/EPR V and iPP/EPR Ti blends was concerned, both the iPP/EPR pairs are to be classified as "negative deviation blends" with GЈ and GЉ values higher than that shown by the plain components. The extent of the observed deviation in the viscosity values and of the increase in the amounts of stored and dissipated energy shown by such iPP/EPR pairs was found to be dependent on copolymer microstructure, being larger for the melts containing the EPR Ti copolymer. The application of the Cross-Bueche equation also confirmed that, in absence of shear, the melt phase viscosity ratio is the main factor in determining the viscosity of iPP/EPR blends and their viscoelastic parameters. The general correlation established between EPR dispersion degree (range of particle size and number-average particle size), as determined in injection-molded samples, and melt phase viscosity ratio () was ratified; the type of dependence of EPR size upon value was in qualitative agreement with the prediction of the Taylor-Tomotika theory. Contrary to expectation, 1-5 for test temperature close to iPP T g , EPR V particles ranging in size between 0.75 and 1.25 m resulted and were