Abstract <p>Phase equilibria in the Ag₈GeSe₆−Ag₇GeSe₅I−GeSe₂ system (system А) were studied by differential thermal analysis (DTA) and X-ray powder diffraction. The <i>T</i>–<i>x</i> diagrams of the Ag₈GeSe₆−Ag₇GeSe₅I and Ag₇GeSe₅I−GeSe₂ boundary binaries, the liquidus surface projection, the 300-K isothermal section, and some polytherms of the phase diagram of system A were constructed. A continuous solid solution series between Ag₇GeSe₅I and the high-temperature cubic Ag₈GeSe₆ phase and limited solid solutions based on the low-temperature Ag₈GeSe₆ phase were found to exist in the Ag₈GeSe₆−Ag₇GeSe₅I boundary system. Solid solution formation decreases the polymorphic transition temperature in Ag₈GeSe₆ and stabilizes the high-temperature cubic phase at room temperature.</p>