Solid liquid phase diagram of the system water/propionic acid was achieved using two methods, the differential scanning calorimetry and a synthetic method. The liquidus curve is a simple eutectic diagram (89 wt % at 243 K) with an "S-shaped" form due to an inflection point (occurring at around 30% in weight of propionic acid), characteristic of a metastable liquid liquid phase separation. The experimental data were modeled using experimental activity coefficients of the same system reported in the literature. The results are coherent, although the working temperature domains are different. The obtained phase diagram shows that ice is in equilibrium with the solution in a large composition domain; this is favorable for our further application of freeze wastewater treatment.