By means of an optimization procedure, the AgNO3–CsNO3–NaNO3 ternary system has been assessed. The mixing enthalpies in the ternary system and the results derived from thermodynamic descriptions for the three limiting binary systems are used. For the AgNO3–NaNO3 and CsNO3–NaNO3 systems, optimized Gibbs energy expressions were taken from the literature. An assessment of the binary AgNO3–CsNO3 system based on the available experimental data is presented in this paper; the calculated phase diagram and thermodynamic functions agree well with experimental data. The parameters describing the Gibbs energies of all binary phases and the ternary liquid are used to predict the phase diagram of the AgNO3–CsNO3–NaNO3 system