Аннотация:The phase diagram of LiF-Li3PO4 system is studied. The eutectic coordinates are 800±5°C, 8±1 mol% Li3PO4. A region of a solid solution based on lithium phosphate with a length of up to 11±2 mol% LiF was discovered. It is assumed that there is heterovalent isomorphism where anionic tetrahedron (PO4)3- is replaced by tetrahedron (LiF4)3-.