The solid-liquid phase diagram of the quaternary system NaCl-FeCl2-FeCl3-H2O has been established at 15 degreesC, under a nitrogen blanket at a pressure of 0.10 mPa, using a synthetic method (variation of conductivity versus. composition). The wet residue and the 'ensemble' methods have been used in complement for the determination of eutectic points. The solubility fields of NaCl, FeCl2. 4H(2)O and FeCl3. 6H(2)O are observed. The sections of the diagram at constant water content are represented in projection as well as the double saturation Lines and the isothermal invariant point (liquid double left right arrow NaCl + FeCl2. 4H(2)O + FeCl3 . 6H(2)O).