This paper presents the reactive extraction of propionic acid from aqueous solution by amine based extractants such as tri-n-octylamine and Aliquat 336, dissolved in a mixture of n-dodecane and 1-decanol. Equilibrium experiments were carried out to investigate the effects of various parameters such as modifier (1-decanol) concentration, extractant type, extractant composition, diluent composition, and initial acid concentration on the extraction efficiency. The extraction efficiency was found to be increased with an increase in modifier composition and extractant composition, and decreased with increases in initial acid concentration. Different biocompatible extractant/diluent systems such as (1) 20% TOA, 20% 1- decanol and 60% n-dodecane, (2) 20% TOA, 30% 1- decanol and 50% n-dodecane, (3) 30% TOA, 20% 1- decanol and 50% n-dodecane and (4) 25% Aliquat 336, 25% 1-decanol and 50% n-dodecane are developed and used in this study. A mathematical model based on mass action law and a population-based search algorithm (differential evolution, DE) is proposed, and is used to estimate the extraction equilibrium constant (KE) and stoichiometry of reactive extraction. Individual equilibrium constants for the simultaneous formation of (1:1) and (2:1) acid:amine complexes are also determined. The extraction system comprised of 20% TOA, 30% 1-decanol, and 50% n-dodecane was found to be the best among the four biocompatible extractant/diluent systems studied. The loading ratios found in the range of 0.113 ~ 1.05 indicated the simultaneous formation of 1:1 and 2:1 complexes between acid and TOA.