Melt-mixed polyblends of high-density polyethylene (HDPE) and an ethylene-propylene copolymer (EPM) containing 78 mol% ethylene were studied in the complete composition range, using quenched and annealed samples. Phase contrast microscopy revealed that the polyblends are heterogeneous, the rubbery copolymer being dispersed as domains with sizes in the micron range, in the HDPE matrix. At intermediate compositions the blend structure was less uniform. DSC studies indicate that crystallization of PE proceeds independently of the rubbery component at all compositions with no evidence of cocrystallization to a significant extent. Dynamic mechanical studies, between –120° and 140°C at 110 Hz, of annealed and quenched specimens confirm that the system is incompatible; however, compound inclusions are formed at low EPM levels. At rubber-rich compositions EPM crystallinity is affected by the presence of the PE phase with the formation of a common crystalline supermolecular structure. Also, a small reduction of the crystallization temperature with an increase of EPM content and a melting point depression of PE at these compositions are evidence for the reduction of crystallite dimensions. This is further corroborated by a decrease of the α relaxation temperature.