A careful evaluation and optimization of all available phase equilibrium and thermodynamic data for the (KF + YF3), (KCl + YCl3) and (YF3 + YbF3) binary systems was performed to obtain optimized model parameters for all solid and liquid phases. After the discussion of four different optimization strategies, the Modified Quasichemical Model in the pair approximation with two short-range ordering compositions near X-YX3 = 0.25 and 0.4 (X = Cl, F) proved to be a suitable method to model the molten salt phase of the (KX + YX3) binary systems. The liquid phase of the (YF3 + YbF3) binary system was considered to be ideal. All thermodynamic data (phase diagram, enthalpy of mixing, activity) that are available for the three binary systems are reproduced within experimental error limits. (C) 2016 Elsevier Ltd. All rights reserved.