in the phase diagram of the system NaF-Na2SO4, a binary compound Na3FSO4 exists. The compound allows to divide the ternary system Na3AlF6-NaF-Na2SO4 into the two subsystems Na3AlF6-NaF-Na3FSO4 and Na3AlF6-Na2SO4-Na3FSO4. Experimental coordinates of the eutectic point in the subsystem Na(3)ALF(6)-NaF-Na3FSO4 are: 7.6 mole%Na3AlF6 + 54.6 mole% NaF + 37.8 mole% Na2SO4, 761 degreesC. Experimental coordinates of the eutectic point in the subsystem Na3AlF6-Na2SO4-Na3FSO4 are: 4.6 mole% Na3AlF6 + 26.3 mole% NaF + 69.1 mole% Na2SO4, 733 degreesC. Thermodynamic model for calculation of the system Na(3)ALF(6)-NaF-Na2SO4 is presented. The thermodynamic model describes well all available thermodynamic data of the system, viz. enthalpies of mixing and solid-liquid phase equilibria.