Because of the the importance of solubility of Cs, we followed the melting temperature of FLiNaK-CsF (5 mol%) and modelled its phase equilibria. We discovered the formation of CsLiF 2 , which melts at lower temperatures than FLiNaK. The formation of CsLiF 2 is predicted by our CALPHAD modeling (Computer Coupling of Phase Diagrams and Thermochemistry); however, its predicted melting temperature is inconsistent with experimental results. Thus, our measurements provide important input information for more accurate modeling of Cs behavior in molten fluoride salts at both microscopic and macroscopic scales.