Organic solutions of a tertiary amine are being used increasingly to separate organic acids from aqueous solutions by reactive extraction. The present paper deals with experimental investigations and the modeling of the liquid-liquid equilibria encountered when toluene, chloroform or methyl isobutyl ketone are used as solvents for the tertiary amine tri-n-octylamine. Acetic acid was used as an example of a carboxylic acid. All measurements were carried out at 298.15 K. Experimental results for the compositions of the coexisting phases are reported. Infrared spectroscopic investigations were carried out to determine the equilibrium constants of some of the chemical reactions observed in the organic solvents toluene and chloroform. The results of the liquid-liquid equilibrium measurements were correlated by a model which takes into account chemical reactions as well as physical interactions. Some of the model parameters were deduced from the spectroscopic investigations and others were determined from literature data on binary subsystems. The remaining parameters (chemical equilibrium constants and physical interaction parameters) were fitted to the liquid-liquid equilibrium data. The final correlation gives a good description of the distribution of acetic acid between the coexisting phases over a wide concentration range. The model also describes reliably the solubility of water in the acetic-acid-containing organic phases and the influence of acetic acid on the solubility of the organic solvents in the aqueous phase.