Tertiary amines are effective extractants for separation of carboxylic acids from dilute solutions. The basic principle of recovery by tertiary amines is through reaction with extraction. Generally amines are used in diluents to improves their physical properties and to provide solvation of the complexes formed by them with the acids. With this context, recovery of propionic acid (present in concentration range of 0.05-0.4 kmol/m(3) in the aqueous phase) by reactive extraction was studied using tri-n-octyl amine (TOA) in different diluents (n-heptane, petroleum ether, ethyl acetate and oleyl alcohol) at 305 K. Extraction using pure diluents was found to be very poor (K-D < 1: except for ethyl acetate). Using the mixtures of extractant-diluents the extraction was significantly improved (K-D increase from: 0.7 to 6.97 for oleyl alcohol; 0.0024 to 0.2783 for n-heptane; 0.0097 to 0.5929 for petroleum ether; and 2.39 to 5.249 for ethyl acetate, using 30% TOA in respective diluents). Results were presented in terms of distribution coefficient, equilibrium complexation constant, overall loading factor, stoichiometric loading factor, modified separation factor and extraction efficiency. Propionic acid-TOA was found to form (1:1) complex with no overloading in any case. TOA-oleyl alcohol system was found to provide the highest extraction. (C) 2008 Elsevier B.V. All rights reserved.