In order to develop new synergistic extraction system of organic acids, the extraction equilibria of organic acids (acetic, glycolic, propionic, lactic, succinic, fumaric. L-malic and itaconic. H(n)A) with tri-n-octylamine (TOA) and/or tri-n-butylphosphate (TBP) were measured. The solvation numbers of the acids were the same as the numbers of the carboxyl groups on each acid molecule in the extraction with TOA or TBP alone. The extraction equilibrium constants were roughly correlated with the hydrophobicity of the acid, except L-malic acid. In the case of using the mixed extractant of TBP and TOA, the synergism was observed in the extraction of all the acids investigated. Specialty, the extractions of glycolic. lactic, succinic and fumaric acids gave a remarkable synergism. The extracted species were estimated to be H(n)A.2TBP.TOA, in addition to H(n)A.(TOA)(n) and H(n)A.(TBP)(n). The calculated results of the distribution ratio of organic acids based on the formation of three complexes explained well the experimental ones.