Geochemical Thermodynamics presents the fundamental principles of thermodynamics for both high- and low-temperature geochemistry. It is intended for first-year graduate students in geology and geochemistry and is a useful prerequisite for more advanced courses in solution-mineral equilibria, petrology and aqueous geochemistry. The authors' aims are to emphasize the basic principles of thermodynamics and to describe these principles within a geochemical framework. Topics of special interest include the phase rule, chemography, solid solutions, ionic activities in mixed aqueous electrolytes, alternative approaches to redox potentials in natural systems and chemical potential diagrams for multiphase systems.