We demonstrate effects of Cs ions on the melting transition and molecular structure of molten FLiNaK (a eutectic mixture of LiF–NaF–KF). FLiNaK is a commonly studied multicomponent model system, which represents the physical and chemical behavior of fluoride salts for nuclear energy applications. Dissolution of nuclear fuels leads to the formation of fission products directly in the molten salt. Cs is one of the most important fission products, due to its relative abundance, long half-life, and potential environmental and health effects. Here, we determine the molecular structure and phase equilibria of dissolved Cs in FLiNaK by a combination of X-ray diffraction, X-ray total scattering, ab initio molecular-dynamics calculations, and computational thermodynamics. Although Cs ions have a relatively large size, we did not find significant evidence that they disrupt the existing molecular structure of the liquid. We found good agreement between our simulated and measured structure factors and calculated that the coordination number of Cs is close to 10. X-ray diffraction in combination with computational thermodynamics demonstrates that upon freezing Cs ions are captured into a CsLiF2 compound, with a lower melting temperature than that of the FLiNaK mixture and a much higher temperature than that predicted for CsLiF2 by computational thermodynamics. We also demonstrated a novel sample environment that we developed to X-ray measurements of molten fluoride or fuel salts.