It is shown by thermal analysis and optical microscopy observations that when an NaCl aqueous solution is cooled as a water-in-oil emulsion, a metastable eutectic is formed that has a eutectic point at around -28 °C. The concentration at which the -28 °C eutectic (solid H2O + solid NaCl) melting peak occurs is found by differential scanning calorimetry to be 0.5-2.5 mol kg-1, with no peak found at concentrations above 3.0 mol kg-1. Furthermore, although the heat of eutectic melting increases when the final temperature during cooling is -49 to -51 °C, this decreases at lower temperatures. The conditions for forming this -28 °C eutectic can be understood based on a phase diagram created from existing research data. This suggests that it is possible to predict the formation of the eutectic under non-equilibrium conditions by using (extending) the hydrohalite (NaCl·2H2O) curve (HH curve) and the NaCl solubility curve (HA curve) obtained under equilibrium conditions.