The equilibrium phase space of the Ag–In–Se system in the part AgInSe2–InSe–Se below 500 K consists seven three-phase regions In2Se3–AgIn11Se17–Se (I), AgIn11Se17–AgIn5Se8–Se (II), AgIn5Se8–AgInSe2–Se (IІІ), In2Se3–In6Se7–AgIn11Se17 (ІV), In6Se7–AgIn11Se17–AgIn5Se8 (V), InSe–In6Se7–AgIn5Se8, and InSe–AgIn5Se8–AgInSe2 (VI). Division of the AgInSe2–InSe–Se into separate phase regions was performed based on electromotive force vs temperature dependences of six electrochemical cells (ECCs) of the type: (−) С | Ag | SЕ | R(Ag+) | PЕ | С (+), where C is the graphite (inert electrode), Ag is the left (negative) electrode, SE is the solid-state electrolyte (Ag3GeS3Br glass), PE is the right (positive) electrode, R(Ag+) is the buffer region of PE that contacts with SE. The process of forming of the thermodynamically stable set of phases from phase non-equilibrium mixture of compounds specified in (I)–(VI) is carried out in the R(Ag+) region. The Ag+ ions act as the small nucleation centers for stable phases. Based on the temperature dependences of the electromotive force of ECCs with PE of the (I)–(VI) phase regions, the standard thermodynamic functions of the binary In6Se7 and three ternary compounds in the adjacent phase regions were calculated for the first time. The agreement of the calculated values of the standard Gibbs energies of the AgIn5Se8 compound in two different phase regions (II) and (V): Δf𝐺(ІІ)○=−(819.6±8.9) kJ·mol −1 and Δf𝐺(V)○=−(820.0±8.9) kJ·mol −1 characterizes the phase composition of the regions (I), (II), (IV), and (V) below 500 K as a combination of compounds of formulaic composition.