Problems of electrolytical preparation of metallic Zr and Hf are considered. Surface tension of fused mixtures of KF-KCl, KF-ZrF4, KF-HfF4, KCl-ZrF4, and KCl-HfF4 is measured by the method of maximum pressure in gas (argon) bubble over a wide range of temperatures and concentrations. The data obtained are presented in the form of linear equations. An adsorption of surfactant and excess free energy, entropy and enthalpy of monomolecular surface layer is computed for 1200 K. Interaction between initial components is revealed.