Phase equilibria in the Cu₈GeSe₆–Ag₈GeSe₆ system were studied by means of differential thermal analysis and powder X-ray diffraction technique as well as electromotive force measurements of concentration chains with the solid electrolyte. The T–x phase diagram and corresponding 'composition–property' diagrams were constructed. It was established that the system is characterized by continuous series of solid solutions between high-temperature modifications of starting compounds. Formation of solid solutions decreases the temperature of polymorphic transitions of both compounds