The PbBi 2 S 4 -PbSnS 2 system was studied by physicochemical analysis methods, and its state diagram was constructed. The system is partially quasi-binary; regions of solid solutions based on PbSnS 2 are determined. At a ratio between the initial components of 1 : 1, congruently melting compound Pb 2 SnBi 2 S 6 forms. The unit cells parameters of Pb 2 SnBi 2 S 6 crystallizing in the orthorhombic system are: a = 15.60 Å, b = 7.80 Å, c = 4.26 Å; space group Pbmm.Bismuth, tin, and lead sulfides are promising semiconductors with photosensitive properties [1,2]. However, the narrow operating temperature range, low melting points, and low mechanical strength limit their practical use. Therefore, study of interactions and phase equilibria in the section PbBi 2 S 4 -PbSnS 2 in the quasi-ternary system SnS-Bi 2 S 3 -PbS is of not only scientific but also practical interest.The compound PbBi 2 S 4 melts incongruently at 980 K [3,4].