Among the various types of phase diagrams, binary aqueous-salt systems are important for studying mineral-fluid interactions, mass transfers, geothermobarometry, etc. Unfortunately, only a few of these diagrams are available, because of the lack of experimental data. A method is proposed to draw an x-T projection from scarce data. As an example, the system H2O-CsCI has been chosen. It is a simple binary system whose univariant curves are incompletely known. Using a solution of CsCl with a salinity close to the roughly estimated eutectic composition, synthetic fluid inclusions were made in order to measure the eutectic temperature by microthermometry (-23.7 °C). Raman spectroscopy confirms that no hydrate exists. A general mathematical model based on a constrained least squares method was set up to fit the two curves, taking into account the eutectic temperature and the triple point of water. The eutectic composition was thus calculated by minimizing the global variance of the model (56.87 wt% CsCl). This example can be extended to other binary aqueous-salt phase diagrams.