The character of the interaction in the InAs2Se4 -InAs2SSe3 system was investigated and its phase diagram was constructed by DTA, XRD, MSA, as well as by measuring the microhardness and determining the density of the alloys. It was found that the InAs2Se4 -InAs2SSe3 section is a quasi-binary section of the In-As-S-Se quaternary system. Solid solutions based on InAs2Se4 at room temperature reach 10 mol % InAs2SSe3, and on the basis of InAs2SSe3 it extends to 15 mol % InAs2Se4. The lattice parameters are calculated from the region of solid solutions based on InAs2Se4 and InAs2SSe3.