Phase equilibria in the ZnS–Ag2GeS3–Ge–GeS2 part of the Ag–Zn–Ge–S system were investigated using differential thermal analysis, X-ray diffraction, and EMF methods. The data was used to model Ag2GeS3–ZnS polythermal section. Further, the mechanism of formation and thermal stability of the Ag2ZnGeS4 compound were established. The results suggest the presence of another quaternary phase Ag4ZnGe2S7 in the temperature range of 695–853 K. The determined phase relations were used to express the chemical reactions. Based on the electromotive force versus temperature measurements, experimental thermodynamic data of the Ag2ZnGeS4 quaternary phase were derived for the first time. The calculated Gibbs energy , enthalpy and entropy values of the Ag2ZnGeS4 compound in both phase regions are consistent, which indicates that Ag2ZnGeS4 has stoichiometric composition.