The system 2PbSe–AgBiSe₂ has been studied by e.m.f. method with solid electrolyte Ag₄RbJ₅. By the analysis of experimental data the formation of wide (to 90% AgBiSe₂) area of solid solutions on the PbSe base is established. From the equations of temperature dependences of e.m.f. the partial thermodynamic functions () of silver in alloys were calculated. On the basis of solid-phase equilibria diagram of system Ag₂Se–PbSe–Bi₂Se₃–Se the potentialformation reactions have been established and standard thermodynamic functions of formation and standard entropy of solid solutions have calculated