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.