Supermolecular structure of blends made of low density polyethylene (LDPE) or isotactic polypropylene (iPP) and ethylene-propylene-diene rubber (EPDM) was studied with X-ray diffraction (WAXS and SAXS), differential scanning calorimetry (DSC) and positron annihilation spectroscopy. Experimental data reveals different behaviour of the plastomers in the elastomer matrix. The crystalline phase of LDPE is to some extent solvated by the amorphous EPDM, whereas addition of iPP enhances the degree of the blend's crystallinity. This additional crystalline phase is of imperfect nature and probably originates from cocrystallization of propylene monomer units from EPDM onto IPP particles. LDPE recrystallizes in the blends with EPDM at a lower temperature, the higher the plastomer content. Contrary to this, iPP recrystallizes in the rubber matrix at higher temperature. The blend's composition influences the morphology of the crystalline phase. Interlamellar amorphous layer thickness slightly increases with an increase of LDPE content, being simultaneously accompanied by a slight decrease of the crystalline lamella thickness. The longs period values remain practically constant. In the case of iPP/EPDM blends, the crystalline lamella thickness increases with an increase of the plastomer content, whereas the interlamellar amorphous layer remains constant, which results in an increase of the long period. The reversed tendency, observed for high-filled samples containing greater than or equal to 50 phr of iPP, is likely to be associated with too many nucleation centres, finally reducing the spherulite size. Despite annihilation taking place in the amorphous as well as the crystalline phase of polymers, the positron lifetime data stays in good agreement with the postulated supermolecular structure of the blends.