The equilibrium for the propionic acid extraction by N1923 in three diluents, represented as proton-donator (n-octanol), polar (butyl acetate), inert (hexane) types, have been studied. The diluents had a significant influence on the equilibrium. The loading ratio, Z, was employed to investigate the effect of diluents on the extraction efficiency. When Zexp was lower than 1, N1923 extraction followed the order: n-octanol>butyl acetate>hexane. When the loading ratio Zexp was higher than 1, the order changed to: butyl acetate>hexane>n-octanol. Equilibrium complex constants K11, K21, K31 have been estimated. The results showed that K11 was much higher than K21 and K31, demonstrated K11 as the dominant factor in the extraction process. Comparing the values of K11 of all the three diluents, the following order was observed: n-octanol>butyl acetate>hexane. The influence of amine concentration on the extraction equilibrium constant in different diluents was also investigated. The extraction equilibrium constant K11 decreased with the increase of amine concentration in n-octanol. When hexane and butyl acetate were used as diluent, the extraction equilibrium constant K11 increased slightly with the increase of amine concentration.