A diagram representing the phase relationships in the Ca3(PO4)2CePO4 phosphate system has been developed on the basis of results obtained using thermal analysis (DTA/DSC/TG), X-ray diffraction (XRD), and scanning electron microscopy (SEM/EDS) methods. One intermediate compound with formula Ca3Ce(PO4)3 occurs in the Ca3(PO4)2CePO4 system at temperature exceeding 1275°C. The compound melts at 1833°C and has one structural form. In the system a solid solution, having the structure of low-temperature form of calcium phosphate Ca3−3xCex(PO4)2−x (where x is a molar fraction of CePO4), exists. The temperature and concentration range of the solid solution has been determined. It has been shown that the Ca9Ce(PO4)7 phase should be understood as a limited solid solution with x = 0.25.