The density and the viscosity of the melts of the system (LiF + NaF + KF)(eut) ( 1) + K2TaF7 ( 2) + Ta2O5 ( 3) have been investigated up to x(3) = 0.02 and x(2) = 0.15. The density of the melts has been measured using an Archimedean method. On the basis of the measured densities, the molar volumes of the melts and the partial molar volume of K2TaF7 in binary (LiF + NaF + KF)(eut) ( 1) + K2TaF7 ( 2) system have been calculated. The viscosity of the melts of the investigated system has been measured using the computerized torsion pendulum method. The viscosity data was fit to modified Redlich-Kister's equation.