The system constituted from the salts KCl, FeCl2, FeCl3 and water is a simple quaternary system. It has been studied at 288.15 K under a nitrogen blanket at a pressure of 0.10 MPa. Solubilities have been determined by conductometric measurements and chemical analysis. The nature of each solid phase in equilibrium with the solution is determined and also the limits of its area of existence. We have observed the crystallisation fields of the KCl, FeCl2.4H(2)O, FeCl3.6H(2)O, and of a double salt 2KCl.FeCl3.H2O. Five adequately chosen, isoplethic sections have been determined at 288.15 K. A projection on the base plane KCl, FeCl2 and FeCl3 of the prism shows curves of constant water content as well as the double saturation lines and the isothermal invariant points. (C) 2003 Elsevier B.V. All rights reserved.