Phase equilibria in the Cu₈GeS₆–Ag₈GeS₆ system were experimentally investigated by means of differential thermal analysis, powder X-ray diffraction techniques, and electromotive force (EMF) measurements of concentration chains with Cu₄RbCl₃I₂ solid electrolyte. The phase diagram and some "composition–property" graphs for title system were plotted. We found that continuous series of solid solutions were formed between high-temperature modifications of starting compounds of the system. Formation of solid solutions causes decreasing of the polymorphic transitions’ temperature for both compounds.