This book presents direct and inverse gas chromatography as a powerful tool for determining a great number of thermodynamic properties and quantities for micro- and especially for macromolecular substances. In order to ensure the continuity and clarity of the presentation, the book first considers some frequently used concepts of chromatography with a mobile gas phase, i.e. the mechanism of separation, retention parameters and the theories of gas chromatography. The employment of this technique as an important method of studying solutions through the most representative statistical models is also discussed. The thermodynamics of direct gas chromatography, as applied to dissolution, adsorption and vaporization underlies the thermodynamic treatment of inverse gas chromatography. The most extensive chapter of the book is devoted to the thermodynamics of inverse gas chromatography and deals with a number of important topics: phase transitions in crystalline-amorphous polymers and liquid crystals, glass transitions, other second order transitions in polymers, the determination of diffusion coefficients, the segregation of block copolymers and other applications.This book is intended for those specialists in research and industry who are concerned with the modification and characterization of polymers, with establishing polymer applications, and with the processing of polymers. It will also be useful to students and specialists interested in the physico-chemical basis of the phenomena involved in gas chromatography in general and its inverse variant in particular.