The equilibrium T − x space of the Ag-Ga-Te-AgBr system in the part Ag₂Te-GaTe-Te-AgBr-Ag₂Te below 600 K has been divided into separate phase regions using the electromotive force (EMF) method. Accurate experimental data were obtained using the following electrochemical cells (ECs): (−) IE | NE | SSE | R{Ag⁺} | PE | IE (+), where IE is the inert electrode (graphite powder), NE is the negative electrode (silver powder), SSE is the solid-state electrolyte (glassy Ag₃GeS₃Br), PE is the positive electrode, R{Ag⁺} is the region of PE that is contact in with SSE. At the stage of cell preparation, PE is a non-equilibrium phase mixture of the well-mixed powdered compounds Ag₂Te, GaTe, Ga₂Te₃, AgBr, and tellurium, taken in ratios corresponding to two or three different points of interest for each of the phase regions. The equilibrium set of phases was formed in the R{Ag⁺} region at 600 K for 48 h with the participation of the Ag⁺ ions. Silver cations, displaced for thermodynamic reasons from the NE to the PE of ECs, acted as catalysts, i.e., small nucleation centers of equilibrium phases. The spatial position of the established phase regions relative to the position of silver was used to express the overall reactions of synthesis of the binary Ga₂Te₅, Ga₇Te₁₀, Ga₃Te₄, ternary AgGa₅Te₈, and quaternary Ag₃Ga₁₀Te₁₆Br, Ag₃Ga₂Te₄Br, Ag₂₇Ga₂Te₁₂Br₉ compounds in the PE of ECs. The values of the standard thermodynamic functions (Gibbs energies, enthalpies, and entropies) of these compounds were determined based on the temperature dependencies of the EMF of the ECs.