The phase diagram of the BiSI–BiSeI system was constructed based on the results of the DTA and XRD measurements. It was determined that it is stable in subsolidus and forms continuous solid solutions between the BiSI and BiSeI compounds. In general, the system is non-quasibinary due to the incongruent melting character of primary compounds. The lattice parameters a-, b-, and c- increase linearly within the homogeneity area of the γ-phase according to Vegard’s rule. Obtained new solid solutions are of interest as optic and high-performance semiconductor materials.