The MgO-Mg-3(PO4)(2)-Mg4Na(PO4)(3)-Na4P2O7 subsystem of the ternary system MgO-Na2O-P2O5 was investigated using thermal analysis (heating and cooling), X-ray diffraction techniques and microscopy in reflected light. Three intermediate compounds: Mg4Na(PO4)(3), MgNaPO4 and MgNa4(PO4)(2) occur in the partial system. Mg4Na(PO4)(3) melts incongruently at 1155degreesC, MgNa4(PO4)(2) melts congruently at 1655degreesC, whereas MgNaPO4 exists only in the solid phase and decomposes at 950-960degreesC. A phase diagram for the partial system is proposed. It has been established that the MgNa4(PO4)(2) phosphate forms a binary system with Na4P2O7. A phase diagram of this system has been constructed. It has been found that a ternary peritectic (tp = 1145 +/- 3.0degreesC) and two ternary eutectics (t(E1) = 984 +/- 3.0degreesC, t(E2) = 732 +/-1.5degreesC) occur in the MgO-Mg-3(PO4)(2)-Mg4Na(PO4)(3)-Na4P2O7 system.