Tertiary amines dissolved in organic solvents are effective chemical extractants for the recovery of carboxylic acids from aqueous solutions as, for example, in downstream processing in biotechnology. Very often those aqueous solutions additionally contain inorganic salts, resulting in phenomena like "salting-out" and ion exchange reactions. In particular, by ion exchange reactions, inorganic salts can be converted to inorganic acids which might have an essential influence on the partitioning of the carboxylic acids. In order to extend a liquid-liquid phase equilibrium model for the reactive extraction of carboxylic acids from an aqueous to an organic solution in the presence of tri-n-octylamine, this contribution deals with the liquid-liquid phase equilibrium in systems of a single mineral acid (hydrochloric, nitric or sulfuric acid) + water + toluene in the presence of tri-n-octylamine. Experimental results are presented for toluene as a model component for the organic phase. Furthermore, the model is presented to correlate such phase equilibrium data. (C) 2003 Elsevier Science B.V All rights reserved.