The method of sorption calorimetry allows simultaneous monitoring of water activity and the partial molar enthalpy of mixing of water. The way of calibration of sorption calorimeter affects the accuracy of the data obtained in sorption experiments. In order to improve the accuracy of the method one can use physico-chemical calibration instead of electrical calibration. The system for use in the calibration should keep its properties constant during the sorption of water, therefore a heterogeneous system can be used. The particular system suitable for this purpose is magnesium nitrate hexahydrate in equilibrium with its saturated solution. The enthalpy of dissolution of the hexahydrate in its saturated solution needed for the calibration has been determined by isothermal titration calorimetry at 25 and 40 °C. The titration results are in agreement with the calculations based on van der Waals differential equation.