Abstract Blends of elastomers are often used as a basis for compound formulations. For instance, tire tread formulations may contain NR, BR and SBR in order to provide cost-effective products with good mechanical properties, dynamic properties and abrasion resistance. Blends of dissimilar EPDM grades are often used when formulating EPDM compounds. In many cases such EPDM-EPDM blends are used to obtain synergy, i.e. properties of the parent EPDM grades are not averaged, but favorable properties of the blend components are combined or unfavorable properties suppressed. In the present paper the miscibility regime for gum stock EPDM-EPDM blends has been calculated based on elementary thermodynamic considerations. Results of calculations have been validated by experiments, and excellent agreement was found. It was found that the chemical composition of EPDM grades has a major influence on miscibility. It is concluded that, as a rule of thumb, two EPDM grades are thermodynamically miscible if the difference between the combined ethylene and third monomer content is less than 12 % by weight. Based on the results it is possible to understand and to some extent predict the behavior of rubber products based on dissimilar EPDM grades.