The extraction equilibrium of acetic acid, malic acid, citric acid and lactic acid with tri-n-octylamine as extractant in toluene were measured at 303 K. Under this experimental condition, all fatty acids were not distributed in toluene. Organic acid, A, reacts with TOA, B, to form ion complexes in toluene. The formed complex species and equilibrium constants were estimated by graphical analysis. The complexes are A(n)B (n=1-6) for acetic acid, (AB)(n) (n=1,5) for malic acid, (AB)(a) (n=1,6) for citric acid, and A(n)B(m) ((n greater than or equal to m) n=1,2,3,4,5/m=1,2) for lactic acid, respectively. The experimental results could be explained by the proposed reaction models and the equilibrium constants were estimated.