The thermochemistry of the ternary system CsI-PbI₂-BiI₃, of interest for applications in photovoltaics, memory devices, and nuclear applications, among other things, is investigated in this work. The binary phase diagrams CsI-PbI₂ and CsI-BiI₃ were subjected to renewed experimental investigation, and the compounds CsPbI₃, Cs₄PbI₆, and Cs₃Bi₂I₉ were found to be the only stable phases in the investigated temperature window. The liquidus lines and invariant equilibria were determined. The phase equilibria in the BiI₃-PbI₂ system were measured for the first time by using Differential Scanning Calorimetry (DSC). The end-members form a solid solution over the entire composition range. The pseudobinary section CsPbI₃-Cs₃Bi₂I₉ of the CsI-PbI₂-BiI₃ ternary system was moreover measured by DSC, as well as the ternary eutectic points. A thermodynamic model of the complete CsI-PbI₂-BiI₃ system was developed by using the Compound Energy Formalism (CEF) for the solid phases and the Modified Quasichemical Model in the Quadruplet Approximation (MQMQA) for the liquid phase. The binary systems were modeled first, and no ternary interaction parameters were found necessary to reproduce accurately the phase equilibria in the ternary system. With our model, the whole liquidus surface of the field CsI-PbI₂-BiI₃ is described for the first time.