An attempt has been made to combine and improve the advantages of different types of reaction calorimeters pursuing the main objectives of simple design and simple data treatment. The calorimeter is operated in the so-called heat-balance mode, i.e. the heal flow through the wall is determined from a heat balance over the jacket, and so is the heat transfer coefficient even if changing during a run. A main new feature is that the jacket can be operated as a once-through differential cooler at high flow rates. The problem of accurately measuring differential temperatures of 0.1 K or below has been solved by applying modern methods of measurement, control and signal processing. A second new advantage to be mentioned is the strictly isothermal operation of both the reactor, by means of a compensation heater, and the jacket, through separate control loops. ＇Strictly isothermal＇ means that the accumulation terms in the heat balances with their ill-defined heat capacities can safely be neglected and error-amplifying differentiation of sampled temperature data be avoided. Some experimental runs to test the performance of this calorimeter are described.