Phase relations in the Cu3SbS4-Sb2S3-S system were determined experimentally over the entire concentration range by means of differential thermal analysis (DTA) and powder X-ray diffraction (PXRD) techniques. One boundary, two internal polythermal sections, and the liquidus surface projection of the system were constructed. Primary crystallization fields of existing phases, as well as, types and coordinates of non- and monovariant equilibria were determined. It was defined that, the concentration triangle under study is an independent subsystem of the Cu-Sb-S ternary system and belongs to the monotectic type with a wide stratification field of two liquids.