Phase equilibria in the Ag8GeS6-Ag8GeTe6 and Ag8GeSe6-Ag8GeTe6 systems were studied using differential thermal analysis and powder X-ray diffraction technique. The T-x phase diagrams were constructed. It has been found that both systems are partially quasi-binary and are characterized by the formation of continuous series of substitutional solid solutions between Ag8GeTe6 and high-temperature cubic modifications of Ag8GeS6 and Ag8GeSe6. When solid solutions are formed, the temperatures of polymorphic phase transitions of these compounds decrease. This leads to the stabilization of the ion-conducting cubic phase in the 40 mol% Ag8GeTe6 range of compositions at room temperature and below. The regions of homogeneity based on Ag8GeS6 and Ag8GeSe6 are 15 and 10 mol.%, respectively. The crystal lattice parameters of the identified solid solutions were calculated from the XRD data, and their linear dependence on the composition is shown