Viscosity is one of structure-sensitive properties of fused salt mixtures. The isotherms of molar viscosity of the RbCl-UCl4 and CsCl-UCl4 fused mixtures for different temperatures as well as variation of activation energy of viscous flow of melts depending on composition reflect the structure changes. The results of measurements of kinematic viscosity and density data of the said fused mixtures are used in computation of the dynamic and molar viscosity and activation energy of viscous flow of binary systems. It is shown that the character of changing in viscosity depending on composition is related to formation of U complex ions.