The SbTeI−Bi 2 Te 3 polythermal section of the phase diagram of the Sb 2 Te 3 +2BiI 3 ↔Bi 2 Te 3 +2SbI 3 reciprocal system was constructed using DTA and XRD methods. It was established that Bi 2-x Sb x Te 3 solid solutions (α-phase) primarily crystallize from the liquid phase along this section. Subsequently, crystallization proceeds by a number of non-and monovariant reactions, α+γ 1 , α+γ 2 , and α+γ 1 +γ 2 (γ 1 and γ 2 are solid solutions based on SbTeI, and BiTeI) heterogeneous regions of are formed in subsolidus.