The viscosity of melts of the system LiF-KF-K2NbF7 has been measured by mea
ns of the computerized torsion pendulum method. The viscosity increases fro
m KF through LiF to K2NbF7. Additivity of logarithms of viscosity was adopt
ed as the ideal behaviour of the mixture. Deviations from such additive beh
aviour in the KF-K2NbF7 binary system as well as in the LiF-KF-K2NbF7 terna
ry one are most probably due to the chemical interaction K2NbF7(1) + KF(1)
double left right arrow K3NbF8(1). The chemical equilibrium of this reactio
n is characterized by the dissociation degree of K3NbF8. At 1100 K it attai
ns the value alpha (0) = 0.45. In the ternary system, statistically signifi
cant ternary interaction does not take place.