By the methods of physicochemical analysis (DTA, XRD, MCA, the definition of density and microhardness) was investigated the nature of chemical interaction in the system InSb₂S₃Se–InSb₂Se₃S and phase diagram constructed. It has been established that the cut is a quasibinary section of the quaternary system In,Sb||S,Se. The system detected the limited fields of solid solutions based on the initial components. It is determined that the solid solutions based on InSb₂S₃Se at room temperature reach up 20 mol. % InSb₂Se₃S, but based on solid solutions InSb₂Se₃S reach up to
12 mol. % InSb₂S₃Se.