A temperature insert for a cooling medium mixing thermostat of internal combustion engines includes, in addition to a first expansion capsule arranged in the mixing space and supported at the housing by way of a piston rod, a second expansion capsule; the first expansion capsule thereby has a higher actuating temperature than the second expansion capsule and carries both a radiator return valve as also a by-pass valve which cooperate alternately with their respective valve seats. The second expansion capsule is exposed to the temperature of the cooling medium in the radiator return line and actuates by way of a common piston rod the radiator return valve and the by-pass valve by movement of the first expansion capsule when the temperature in the radiator return line reaches the actuating temperature of the second expansion capsule. As a result the regulating temperature of the cooling medium fed to the engine is lowered at higher outside temperature and/or engine load compared to the curve resulting from the function of the first expansion capsule.