We report on a technique for monitoring the crystallization of water and aqueous solution of NaCl at atmospheric pressure, when cooled via liquid nitrogen, using a Fresnel reflection-based optical fiber sensor. The crystallization of distilled water and the associated changes in refractive index inferred from the sensor response comply with the previous reports on physical properties of supercooled water. The phase separation of NaCl.2H2O and the formation of eutectic mixture were inferred from the distinct features of the sensor signal during the cooling of NaCl solution. But the thermocouple did not detect the exothermic heat of crystallization due to rapid cooling. The influence of temperature gradients while interpreting the optical signals during this rapid cooling process and the effects of sensor debonding during the heating phase are discussed. The results demonstrate the potential of Fresnel sensors for monitoring the crystallization-induced phase changes in supercooled salt solutions and offers applications in areas where monitoring and controlling crystallization is important. © 2016 Optical Society of America.