Phase equilibria in the ZnS–Ag 2 GeS 3 –Ge–GeS 2 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 Ag 2 GeS 3 –ZnS polythermal section. Further, the mechanism of formation and thermal stability of the Ag 2 ZnGeS 4 compound were established. The results suggest the presence of another quaternary phase Ag 4 ZnGe 2 S 7 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 Ag 2 ZnGeS 4 quaternary phase were derived for the first time. The calculated Gibbs energy, enthalpy and entropy values of the Ag 2 ZnGeS 4 compound in both phase regions are consistent, which indicates that Ag 2 ZnGeS 4 has stoichiometric composition.