Physicochemical properties of the system (FLiNaK)(eut) (1) + K2NbF7 (2) were investigated. Phase equilibria were studied by a thermal analysis method. Density of the investigated system was measured by the Archimedean method. On the basis of these data, molar volumes were calculated for selected temperatures, and the partial molar volume of K2NbF7 in (FLiNaK)(eut) was evaluated adopting the value (V) over bar (2)/cm(3).mol(-1) = (121.1 +/- 0.4) at 1073 K, indicating volume contraction. Surface tension was measured by the maximal bubble pressure method, and viscosity was measured using the computerized torsion pendulum method.