Based on viscosity measurements of seven electrolyte soultions (KCl, K Br, KI, LiCl, Na2CO3, Li2SO4, and MnSO4) in heavy water and the viscosity data of four electrolyte solutions (KCl, KBr, Kl, and Na2CO3) in ordinary water the activation energies of viscosity have been determined from the Pantchenkov equation. The dtermination were made at 25°, 35°, 45°, 60°, 75°, and 90°, at various concentrations up to near saturation. The activation energies of viscosity for pure heavy and ordinary water have been determined, too. The results allowed conclusions on the various influences of the investigated salts on the structure of heavy and ordinary water solutions.