Abstract Because amorphous cis-1,4-polyisoprene(CPI) and crystalline trans-1,4-polyisoprene(TPI) are isomers, with similar refractive index, electron density and even glass transition temperatures, confirmation of miscibility is difficult by using most direct methods, such as phase contrast optical microscopy, small angle X-ray scattering (SAXS) and differential scanning calorimetry (DSC). In this study, a new perspective on liquid-liquid phase separation affecting subsequent crystallization based on the converse thinking is proposed to assess the miscibility of CPI and TPI. First, the TPI crystallization is investigated with the aid of multiple techniques such as DSC, SAXS, and atomic force microscopy (AFM), and the miscibility is deduced based on the results. Furthermore, dynamic rheology is used here on CPI/TPI blends for the first time to analyze their rheological response for the confirmation of their compatibility. All results show that CPI and TPI are thermodynamically compatible in a wide range of compositions when they are in the molten or solid state.