Phase equilibrium is a result of intermolecular forces. When molecules approach each other, intermolecular forces can result in the formation of short living adducts. With increasing strength the life time of those adducts increases, finally resulting in new, stable molecules. Therefore the effect of strong intermolecular forces on the thermodynamic properties can also be described by chemical reactions. The fundamental relations governing phase equilibria in chemical reactive mixtures are shortly reviewed and some examples for industrially important systems where chemical reactions have an essential influence on phase equilibria are discussed.