Processes for the preparation of a thermoplastic elastomer comprising a triblend of engineering resin, dynamically vulcanized alloy of thermoplastic olefin polymer and elastomeric copolymer, and a compatibilizer for the engineering resin and alloy. The compatibilizer can be formed in a separate step or in-situ in a one step process. The processes produce compositions which retain structural integrity at high temperature, and have good tensile properties, elongation and fluid resistance.