Propionic acid extraction from the aqueous phase is important in view of its number of uses. The recovery technique should provide high selectivity and yield. A number of techniques like distillation, membrane, dialysis, electrodialysis, and reactive extraction are available. The profound success of reactive extraction has been noted in the recovery of carboxylic acids from dilute solutions. In the present paper, equilibrium with propionic acid using tri-n-butyl phosphate, an organophosphorous compound, in eight different diluents (1-dodecanol, benzene, toluene, heptane, hexane, butyl acetate, petroleum ether, and paraffin liquid) has been studied. Equilibrium parameters such as distribution coefficients, loading ratio, degree of extraction, and equilibrium complexation constants have been presented. The data obtained are useful in understanding the equilibrium characteristics and efficient design of the recovery process of propionic acid by reactive extraction.