The phase equilibria of the Ag–Ge–Sb–Te system in the part Ag8GeTe6–Ge–GeTe–Sb2Te3 were investigated by the electromotive force (EMF) method. The determined phase relations were used to express the chemical reactions. The potential-forming reactions were performed by applying electrochemical cells (−) C | Ag | Ag2GeS3-glass | D | C (+), where C is graphite, Ag2GeS3-glass is the fast purely Ag+ ions conducting electrolyte, and D is an equilibrium mixture of phases. According to the experimental data on the EMF versus temperature of each electrochemical cells, analytical equations for the Gibbs energies of GeSb8Te13, GeSb6Te10, GeSb4Te7, GeSb2Te4, Ge2Sb2Te5, Ge3Sb2Te6, and Ge4Sb2Te7 compounds were obtained. The thermodynamic properties of silver-saturated tetradymite-like compounds have been calculated for the first time. A good correspondence between experimental values and structure data reported in the literature has been established.