The phase diagram of the system CePO4-K3PO4 has been determined based on investigations by differential thermal analysis, X-ray powder diffraction. IR spectroscopy and optical microscopy. The system contains only one intermediate compound K3Ce(PO4)(2), which melts incongruently at (1500 +/- 20)degreesC. This compound is stable down to room temperature and exhibits a polymorphic transition at 1180 degreesC. It was confirmed that the low-temperature form beta -K3Ce(PO4)(2) crystallizes in a monoclinic system, space group P2(1)/m with unit cell parameters a = 9.579 (5), b = 5.634 (6). c = 7.468 (5) Angstrom; alpha = gamma = 90 degrees, beta = 90.81 (3)degrees; V = 403.083 Angstrom (3).