Surface tension of fused salt mixtures of KF-KCl-HfCl4 is studied depending on temperature and concentration. The results are used for optimization of electrolytic preparation of metallic hafnium. The dependences of excessive surfacial enthalpy and entropy are identical. This points to the fact that the bounden energy makes a decisive contribution into general energy of surfacial layer. The isotherms of sections testify considerable interaction of components in the fused mixtures.