The geochemical modeling based on thermodynamic equilibrium has become a well recognized tool for understanding the processes of water-rock interaction and its mechanisms. However, in the last 40 years, has paid little attention to rock-water-gas interaction (heterogeneous multi-component systems) in geothermics (perhaps due to the complexity of the calculations), where the water is only one of these components of the vapor-liquid phases containing a mixture of reactive and inert gases. In this sense, this book is useful for modeling geothermal reservoirs and geochemical interactions through thermodynamics of liquid-vapor phase equilibria, particularly with the development of an equation for Poynting correction that can correct the deviation from ideality that show at high pressures the liquid-vapor systems. The objective of this book is present a speciation and thermodynamic equilibrium calculations method of vapor-liquid multicomponent phases below the critical temperature of water (647 K) and pressures up to 2 kilobar with computational tools.