The properties of melt-mixed polyblends of high-density polyethylene (HDPE) and an amorphous ethylene-propylene-diene terpolymer (EPDM), containing 62 mol% of ethylene, were studied. The compositions covered the complete range. The techniques used were differential scanning calorimetry, dynamic mechanical spectroscopy, and phase contrast microscopy. The latter technique revealed a heterogeneous structure with a finer dispersion of the rubbery particles (at low rubber compositions), compared to a blend of a HDPE/ethylene-propylene copolymer (EPM) having a higher ethylene content and of low crystallinity. The dynamic mechanical technique at isochronous conditions (110 Hz), between–120° and 140°C, confirmed the heterogeneous nature of the polyblends and revealed mixed-phase formation at all rubber compositions. Appropriate mechanical models tested at -25°C revealed that the HDPE phase forms the matrix of the polyblends for volume fractions of EPDM, Φ (EPDM) < 0.8. More severe tests over an extended temperature range, using other models, led to the conclusion of a basically incompatible system of noninteracting phases with a highly parallel connectivity. This is consistent with a laminar distribution of phases. DSC studies showed that at Φ (EPDM) 8gt; 0.2, the PE crystallization mechanism changes, but there was no evidence of cocrystallization. In addition, the reorganization of the PE crystals during melting is significantly influenced by the viscous rubber surrounding it. This, together with the reduction of the α PE relaxation temperature, is taken as evidence that a limited mixing of the amorphous phases takes place within the interlamellar region, accompanied by a decrease of the crystal fold period.