<p>Phase equilibria in the Ag₂Se—Ag₈SiSe₆—Ag₈SnSe₆ system were studied by differential thermal analysis (DTA), X-ray diffraction analysis (XRD), and scanning electron microscopy (SEM). The diagram temperature—composition of the Ag₈SiSe₆—Ag₈SnSe₆ boundary system, a number of polythermal sections, isothermal sections of the phase diagram at 300, 1100, and 1150 K, and projection of the liquidus surface were constructed. The Ag₈SiSe₆—Ag₈SnSe₆ side system is quasi-binary and characterized by the formation of a continuous series of solid solutions (δ-phase) between high-temperature cubic modifications of the initial compounds. When solid solutions are formed, the temperatures of polymorphic transformations of both compounds decrease, which leads to the stabilization of the ion-conducting cubic phase in a composition range of 10–65 mol.% Ag₈SnSe₆ at room temperature. Rather narrow (2–3 mol.%) regions of solid solutions based on the low-temperature and intermediate modifications of Ag₈SiSe₆ were revealed, whereas up to 20 mol.% solid solutions based on the low-temperature modification of Ag₈SnSe₆ were identified. The liquidus surface of the system under study consists of two fields corresponding to the primary crystallization of the δ-phase and solid solutions based on high-temperature Ag₂Se. Phase transitions associated with polymorphism of the initial compounds and related phases based are observed in the subsolidus part of the system.</p>