The viscosity of melts of the system LiF + NaF + K2NbF7 has been measured by means of the computerized torsion pendulum method. The viscosity increases from LiF through NaF to K2NbF7. Deviations from the additivity of logarithms were considered. The measurement of viscosity in the investigated system has shown that a significant ternary interaction exists in the melts, which can be ascribed to the formation of the [NbF8](3-) anions.