The Sn – As – P system is characterized by the presence of unlimited solid solutions (Sn4As3)x (Sn4P3) 1–х (α-phase) and solid solutions based on tin monoarsenide (β-solid solutions) and SnP3 phosphide (γ-solid solutions). The solubility of phosphorus in SnAs depends not only on temperature, but also on the direction of the polythermal cross section, i.e. from that in the form of an individual simple substance or in the form of any of the tin phosphides, the doping component is introduced. In the present work, the confi guration of the region of existence of solid solutions of phosphorus in tin monoarsenide was investigated. Alloys of SnAs – Sn4P3:SnAs – Sn0.43P0.57; SnAs – SnP3 and SnAs – P polythermal sections, annealed at 753 K, were studied by X-ray phase analysis, the concentration range of the existence of a homogeneous solid solution was established, the parameters of the crystal structure were determined. The construction of quasi-chemical schemes for the doping of tin monoarsenide showed that with the introduction of SnP3, the greatest deviation from stoichiometry towards the lack of tin is observed. The highest solubility of phosphorus, reaching 30 mol % corresponds to this cut. Thus, the region of existence of solid solutions based on tin monoarsenide has an asymmetric shape shifted toward an excess of volatile components, and the maximum solubility of phosphorus corresponds to the direction of the polythermal section SnAs – SnP3. Taking into account the phase equilibrium scheme of the Sn – As – P ternary system and the obtained data on the confi guration of the region of existence of homogeneous solid solutions of phosphorus in the tin monoarsenide, a phase subsolidus demarcation scheme of the phase diagram of this system was carried out.