The solubility behavior of various elastomer additives, i.e. processing promoters (PRMs) was studied in EPDM, NBR and PP by using thermoanalytical and dynamic-mechanical methods. Binary rubber/thermoplastic blends containing selected PRM types were then investigated with regard to phase morphology, compatibilization and related final properties. The incorporation of some additives resulted in significant effects on the physical characteristics of the mixtures. It was demonstrated that the PRMs act as effective homogenizing agents, contributing to considerably improved dispersion and compatibility of the blend components.