The First All-Union Symposium on Phase-Separation Phenomena in Glasses was held in Lenin­ grad from April 16 to 18, 1968. The symposium was organized by the Section of the structure of glass and the nature of the glassy state of the Scientific Council for the problem "New Tech­ nical Materials and Coatings Based on Glass and Refractory Compounds" of the State Committee for Science and Technology of the Council of Ministers of the USSR, by the Scientific Council for the problem "Physicochemical Principles of Production of New Heat-Resistant Inorganic Mate­ rials" of the Division of Physical Chemistry and Technology of Inorganic Materials, Academy of Sciences of the USSR, by the Leningrad regional board of the D. 1. Mendeleev All-Union Chemi­ cal Society, and by the 1. V. Grebenshchikov Institute of Silicate Chemistry, Academy of Sciences of the USSR. The present collection consists of the Proceedings of the Symposium and is No. 3 in Vol. 5 of the series "The Glassy State," published by the Academy of Sciences of the USSR since 1955. The book contains the results of theoretical and experimental investigations of phase sep­ aration of glasses into two glassy phases, and of the influence of this submicroscopically heter­ ogeneous structure on certain properties of glass. Metastable phase separation in glasses was discovered about 10 years ago.