The surface tension of the molten system KF(1) + K2NbF7(2) + Nb2O5(3) UP to x(3) = 0.2 has been investigated using the maximum bubble pressure method over the temperature range from 1133 K to 1233 K. The uncertainty of the surface tension measurement was estimated to be +/- 1.5%. On the basis of the data presented, the composition dependence of the surface tension and the surface tension excess were calculated.