The enthalpies of formation of liquid AlF3-BaF2-NaF ternary mixtures were measured by high-temperature calorimetry. The "direct drop" mixing procedure was applied in accordance with the physico-chemical properties of the system. The heat of mixing, referred to the liquid state, requires a knowledge of the enthalpy change between room temperature and the experimental temperature of the samples added to the melt. This term is not well known for AlF3, because it sublimes under normal conditions. Therefore, in this paper we used a new method to analyse raw experimental data based on the use of a thermodynamic model which evaluates the excess functions of the liquid, The results obtained are compared with literature values.