Calorimetric measurements of sodium chloride dihydrate NaCl·2H2O (mineral name hydrohalite) were carried out with using DSC. Heat capacity from 190 to 250 K was measured and found to increase from 109 to 137 J mol−1 K−1. The enthalpy of formation of hydrohalite from solid ice and halite at 273.15 K was derived from the thermal effect of melting/decomposition in DSC measurements and found to be close to − 1.8 kJ mol−1. The same DSC results show clearly that the upper temperature limit for the existence of hydrohalite is several degrees greater than the current value of 273.15 K accepted for the peritectic decomposition of hydrohalite. The phase diagram of the NaCl–H2O system needs correction.