AbstractThe dynamic mechanical behavior of the binary blends high‐density polyethylene/EPDM rubber (ethylene‐propylene‐diene terpolymer) and isotactic polypropylene/EPDM rubber was investigated by means of a free‐oscillating torsional pendulum. Comparison of the moduli of the systems studied and predictions drawn from several models show that phase inversion in the polyethylene/EPDM blends takes place at the volume fraction vPE = 0.5, while polypropylene preserves a certain phase continuity also as the minor component (0.5 > vPP > 0.2) in blends with EPDM. Because of faster shrinkage in the course of cooling, inclusions are exposed to negative hydrostatic pressure, which accounts for a drop in the glass‐transition temperature of the rubber. The strength of the relaxation associated with the glass transition of inclusions and tensile impact strength increase proportionately to the rubber content in polypropylene (0 < vEPDM < 0.4). Dynamic mechanical response spectra of commercial EPDM/polyethylene impact modifiers are compared with the results for model systems.