Using a numerical Gibbs free-energy minimization technique, the activity coefficients of actinide and rare-earth chlorides in molten LiCl/KCl eutectic at 450 degrees C have been calculated. Laboratory tests of an electrochemical process for separating actinide metals from rare-earth metals in LiCl/KCl solvent at 450 degrees C provided experimental concentration and cathode potential data. The generalized expansion by Wohl was used to express the concentration dependence of the excess Gibbs free energy. The activity coefficients were expressed in terms of the Wohl volume and interaction energy parameters. The Wohl parameters for the activity coefficient expansions were obtained by minimizing the total Gibbs free energy expressed in terms of the experimental mole fractions. This thermodynamic model will be valuable for process design and scale-up calculations.