Differential thermal analysis up to complete melting was thoroughly performed for the pseudobinary system LiF–BiF 3 . Melts with high bismuth fluoride concentration suffered from high evaporation; nevertheless even pure BiF 3 could be molten at 655°C. The system contains one intermediate compound BiLiF 4 which melts by peritectic decomposition with the formation of LiF at 450°C. The eutectic between BiLiF 4 and BiF 3 melts at 415°C. Thermodynamic assessment yielded the following parameters for BiLiF 4 : ΔH=−1, 514, 900Jmol −1 , S=158.5Jmol −1 K −1 , c p =166.173−0.01072·T (in Jmol −1 K −1 ). The melt has negative excess Gibbs free energy.