A phase diagram of the Rb3Y(PO4)(2)-BaRbPO4 system for the whole range of compositions, up to a temperature of 1700 degrees C, has been developed on the basis of the results of thermal and XRD analyses and microscopic examinations. The investigated system has an eutectic character. Limited solid solutions with the high-temperature () and low-temperature () structure of BaRbPO4 form in this system. The maximum solubility of Rb3Y(PO4)(2) in the high-temperature solid solution ((ss)) occurs at the eutectic temperature (1275 degrees C), amounting to similar to 80 mass% Rb3Y(PO4)(2). At 988 degrees C, the (ss) decomposes according to eutectoidal reaction: (ss) (s) + Rb3Y(PO4)(2). The maximum content of Rb3Y(PO4)(2) in the low-temperature limited solid solution ((ss)) does not exceed 10 mass% Rb3Y(PO4)(2). The (ss) phase has a hexagonal glaserite-like structure with the unit cell parameters a approximate to 5.7 and b approximate to 8.1 angstrom.