The dynamic mechanical behavior of ternary blends of isotactic polypropylene (80–0 percent)/EPDM rubber (20 percent)/high‐density polyethylene (0–80 percent) was investigated in the temperature range from −196 to 100°C by means of a free‐oscillating torsional pendulum. The structure of the blends was examined by a scanning electron microscope on etched surfaces cut by a fractured glass edge in liquid nitrogen. Dynamic mechanical response spectra and microphotographs of the systems studied show that the minor thermoplastic forms the core of EPDM rubber inclusions. At 20 percent rubber in the blends, the inclusions can accommodate from 20 to 30 percent polyethylene or polypropylene. Addition of either thermoplastic not exceeding this limit has almost the same effect on the stiffness, damping, and yield stress of the blends as the addition of the same amount of rubber. Ternary blends with equal or slightly different polypropylene and polyethylene fractions have the structure of interpenetrating phases in which EPDM rubber forms the interface layer.