It is the purpose of this book to present a concise and sufficiently detailed de scription of the present state and possibilities of calculating chemical equilibria of gas mixtures. It is based on a book by one of the authors, published in Czech by the Publishing House Academia in Prague. The rapid development of the topic during the two years since publication of the Czech edition has made it necessary to revise practically all the sections in order to bring them up to the present level of know ledge. One reason for writing this book was the practical requirement of contemporary industry, where a rational utilization of equilibrium composition calculations may provide valuable information concerning processes under study in all stages of their implementation. A second reason was the need of a text-book for studying this part of chemical thermodynamics in the scope as taught at the Institute of Chemical Technology, Prague. These two basic motives determine the overall structure of the book, as well as the proportions and arrangement of the chapters. The book includes fundamental thermodynamic concepts as well as the mathematical apparatus needed to solve the problems involved, care being taken that the discussion should always l!!ad to a practical procedure of performing equilibrium calculations in gas-phase systems of any degree of complexity whatever. Knowledge of chemical thermodynamics on the level of a fundamental university course is assumed.