The book contains general issues of solution thermodynamics: partial and integral functions, ideal, regular and athermal solutions, the influence of various factors on solubility, critical phenomena in solutions, as well as special sections (solutions at high pressures, thermodynamics of solid phases of variable composition, elements of the theory of "ordering solid solutions and statistical thermodynamics of solid phases of variable composition). The book is designed for senior students of chemical and metallurgical specialties, graduate students and scientists, as well as engineers interested in solution thermodynamics. To read the book, preparation within the framework of the general course of physical chemistry is sufficient.