A calorimeter is described for the simultaneous measurement of the heat capacity and length changes of absorbent‐adsorbate systems between +25 and −45°C. The heat capacity of sodium chloride between 0 and −28°C is given and is found to agree with previous results within 0.6%. The calorimeter is also suitable for studies of heats of adsorption and solid state reactions such as hydration and phase transition.