The formation Gibbs energies of molten K2NbF7, K2NbOF5, and K2NbO2Fe3 at the temperature of 750 degreesC were calculated from the previously performed voltammetric and chronopotentiometric measurements in the system LiF-NaF-K2NbF7-Na2O [1-3] and from the equilibrium constants of reactions leading to the formation in the melts of the individual oxyfluoroniobates. For the formation Gibbs energy of K2NbF7 the average value Delta (f)G(0)(K2NbF7) = -(1340 +/- 25) kT mol(-1) was obtained. For K2NbOF5 the formation Gibbs energy is Delta (f)G(0)(K2NbOF5) = -(1526 +/- 16) kT mol(-1). This value may, however. not be exactly correct since the product of the electrochemical reduction was not pure niobium, but the solid solution of oxygen in niobium. For the formation Gibbs energy of K2NbO2F3 the value Delta (f)G(0)(K2NbO2F3) = -(1100 +/- 100) kJ mol(-1) was calculated. This value may be charged by a substantial error as well due to the probably incorrect value of the formation Gibbs energy of K2NbOF5.