Argyrodite-family compounds are environmentally friendly functional materials that exhibit valuable properties, including thermoelectric, photoelectric, and optical characteristics, as well as significant ionic conductivity owing to the high mobility of Ag⁺ and Cu⁺ ions. In this study, the phase equilibria in the Cu8SiS6–Cu₈SiSe₆ argyrodite system were investigated using differential thermal analysis, X-ray phase analysis, scanning electron microscopy, and microhardness measurements. It was established that the system is quasi-binary in nature and is characterized by the formation of a continuous solid-solution series (δ-phase) between the high-temperature modifications of the initial compounds, whereas broad solid-solution regions with extensive homogeneity ranges form based on their low-temperature modifications. The formation of solid solutions is accompanied by a decrease in the polymorphic transformation temperatures of the initial phases, resulting in the stabilization of the high-temperature δ-phase at room temperature over a broad composition range of 30–75 mol% Cu₈SiSe₆. The crystal lattice parameters of the synthesized solid solutions were determined, and their linear dependence on composition in accordance with Vegard’s law was established.