Metastable eutectic melting of solid mixture NaCl + H2O was measured in a wide range of its compositions by using a DSC. The new eutectic melting was found to occur near −27 °C, below the well know ice-NaCl∙2H2O eutectic point (−21.1 °C, 23.3 wt.%) of equilibrium both phase diagram. The new low-temperature eutectic was detected for all samples, in two-phase regions, Н2О-NaCl∙2H2O and NaCl∙2H2O-NaCl. The new eutectic temperature is close to the value of ∼−28 °C reported by Roedder (1984) for the mixture of solid H2O and NaCl, which was never repeated or reaffirmed independently since then. The heat effect of the metastable eutectic melting did allow us to estimate the enthalpy of formation of hydrohalite from salt and ice: NaCl(cr) + 2H2O(cr) = NaCl∙2H2O(cr) + ΔH (= −0.73 ± 0.07 kJ mol−1).