A detailed study of phase equilibria of mixed fluorides in the BaF2-YF3 system was conducted. For this study, a series of compounds with the general formula Y1-xBa2+1.5xF7 (-1.33 less than or equal to x less than or equal to 1.0) were synthesized and characterized by powder X-ray diffraction (XRD). XRD analysis revealed that there exists an extensive solubility range of YF3 in a BaF2 system (approximate to 25 mol%), up to the composition Y0.8Ba2.30F7 (x = 0.2). Up to this composition, the BaF2 structure (cubic fluorite) persists. Compositions closer to YF3, however, manifested an ordered fluorite-derived structure with a rhombohedral (hexagonal) unit cell. This ordered phase begins to form at Y1.0Ba2.15F7. At Y1.0Ba2.0F7, the composition is a biphasic mixture of the cubic fluorite and the rhombohedral structures. This investigation supports the formation of the earlier reported Y2BaF8 (monoclinic) compound, which we have demonstrated to be a very stable phase in this system. On either side of Y2BaF8 (i.e., towards BaF2 or YF3), there coexist the additional phases rhombohedral and O-YF3, respectively. The Y2BaF8 exists as a line compound in this system.