The first quantitative studies of the thermodynamics of hydrothermal solutions were made in 1903 by A.A. Noyes and W.D. Coolidge (1903), who used the change in conductance associated with ionization to measure the ionization constants of water and aqueous acids and bases up to about 300 °C at steam saturation. The need to develop nuclear reactor coolant chemistry in the 1950’s led national laboratories in several countries to develop new experimental methods for measuring solubility products, equilibrium constants and activity coefficients under the corrosive conditions encountered at elevated temperatures. Complementary studies by geochemists investigating geothermal systems and ore body formation have led to additional experimental techniques suitable for near critical and supercritical conditions (See, for example: Palmer et al. 2004; Mesmer et al. 1997; Ulmer and Barnes 1983). Modern methods include potentiometric titrations, conductivity, solubility, UV-visible Raman and infrared spectroscopy, X-Ray absorption spectroscopy (XAS), neutron scattering, and the calorimetric methods that are the subject of this review. These methods are all important to hydrothermal geochemistry and industrial chemistry because they yield thermodynamic parameters and structural information about the aqueous reactions associated with specific systems under study. However, it is simply not practical, or even possible, to measure the properties of all the thermodynamic and kinetic parameters that control important hydrothermal processes. This problem has been addressed by the development of thermodynamic databases and geochemical modeling codes, which can be used to predict the stability of mineral assemblages, vapor-liquid equilibria, and the effects of solubility and redox reactions on mass transport as a function of pressure and temperature under hydrothermal conditions (See, for example: Anderko et al. 2002; Oelkers et al. 2009). Indeed, the emergence of process simulation software, based on multi-component, multi-phase chemical equilibrium models, as a tool for interpreting metamorphic systems, chemical process design, and …