High temperature molten salts are very important in the industrial field such as active metal production. In any case, including the use as the electrolyte, the physical properties, for example, density, electrical conductivity, viscosity are essentially important. On the other hand, molten salts consist of ions and are agglutinated by the coulombic interaction. The ionicity of the molten salts is affected by the strength of coulombic force of the constituent ionic species. Too strong coulombic force makes the salts non ionic, for example, SiCl4, and prevents the formation of stable ionic molten salts. Therefore, the phenomena of complex formation are important to consider the properties of molten salts. The author studied various physical properties of many molten salts. In this paper, the physical properties are described related to the ionic constitution including complex formation in the molten salts.