Abstract: An apparatus for determining the thermal efficiency of a chemical reaction wherein a reaction vessel is equipped with a double-wall jacket or shell which forms a heat exchanger, a heat transfer fluid medium is circulated so rapidly that the difference of its temperature at the inlet and at the outlet of the shell throughout the entire duration of the reaction, with the exception of possibly occurring momentary irregularities in the reaction kinetics, does not exceed 1.degree. C. A temperature regulator is provided which is constructed as a mixture regulator and encompasses a respective container for heat transfer fluids which are hotter and colder with respect to the temperature of the heat transfer fluid which is circulated in the circulation system and each such container is operatively connected with the circulation system as a function of the reference value deviation of the temperature of the reaction mixture.