The Sb–S and In–S binary systems were assessed thermodynamically using the CALculation of PHase Diagrams (CAL-PHAD) approach based on the experiment data in the literature. Both phase diagrams revealed a congruent melting compound and liquid immiscibility. Therefore, associate species, Sb 2 S 3 and In 2 S 3 , were introduced into the associate model to describe the liquid phase during optimization. The binary intermediate compounds, Sb 2 S 3 , InS(α, β), and In 6 S 7 , were treated as stoichiometric phases. Considering the wide composition range, In 2 S 3 (α, β, γ), were modeled using the sublattice model. A set of self-consistent thermodynamic parameters representing most of the reliable thermodynamic properties and phase diagram information were derived.