Reactive extraction by a water insoluble amine is a common process for the recovery of carboxylic acids from aqueous phases. The amine which is usually dissolved in an organic solvent forms weak complexes with the carboxylic acids. The complexes are insoluble in water and therefore, the acids are extracted from the aqueous into the organic phase. The basic design of the extraction process requires a thermodynamic framework for the liquid-liquid equilibrium. Such phase equilibrium is characterized by rather uncommon and surprising effects, which must be described and (if possible) predicted by the thermodynamic model. The present contribution summarizes the research by the author's group over 15 years in that field. (c) 2005 Elsevier B.V. All rights reserved.