High-temperature near-IR absorption spectroscopy was used to study the dissolution of UO2 in molten MgCl2, CaCl2, and AlCl3 melts. The study reveals that UO2 is most soluble in molten AlCl3, followed by the melt of MgCl2. The solubility of UO2 in molten CaCl2 is too small to be measured with optical spectroscopy. This strong dependence of the solubility on the cations of the melts was rationalized by the use of the Flood-Forland-Grjotheim thermodynamic cycle.