Semi-interpenetrating polymer networks based on two elastomers, cis-1,4-polyisoprene (PI) and thermoplastic polyurethane elastomers (TPUs) were prepared in varying compositions. The PI component was cross-linked using peroxide initiators. Modulus and mechanical properties were investigated as a function of composition and temperature. Slight synergisms were observed in mechanical properties, particularly for compositions containing 10% PI by weight. Little or no molecular mixing is shown by differential scanning calorimetry (DSC) for these two-phase materials.