A complete critical evaluation of all available phase diagram and thermodynamic data has been performed for all condensed phases of the (MgCl2 + CaCl2 + MnCl2 + FeCl2 + COCl2 + NiCl2) system, and optimized model parameters have been found. The model parameters obtained for the binary subsystems can be used to predict thermodynamic properties and phase equilibria for the multicomponent system. The Modified Quasichemical Model was used for the molten salt phase, and the (MgCl2 + MnCl2 + FeCl2 + COCl2 + NiCl2) solid solution was modeled using a cationic substitutional model with an ideal entropy and an excess Gibbs free energy expressed as a polynomial in the component mole fractions. This is the first of two articles on the optimization of the (NaCl + KCl + MgCl2 + CaCl2 + MnCl2 + FeCl2 + COCl2 + NiCl2) system. (C) 2004 Elsevier Ltd. All rights reserved.