The physical properties of water and steam have been the subject of lhorough investigation for a long time. It can, on the one hand, be due to the important role of this substance in the processes that take place in nature and, on the .other hand, due to its wide industrial use. Steam is the most important working substance used in conventional and nuc lear power plants, in chemical engineering and other fields of industry. The variety of practical use of water and steam generates a need for knowing their thermodynamic, transport, electrical, and other properties over a ·very wide range of temperatures and pressures. International Conferences on the Properties of Steam, which have taken place since 1929, have the aim to promote wide exchange of the results of theoretical and experimental studies into the properties of ordinary and heavy water in a11 phase states. In the course of last decade f.he investigation oC physical and chemical properties of concentrated and dilute aqueous solutions that are of interest in the first place from the point of view of power play an important part in these studies.