The equilibria of organic acids (acetic, glycolic, propionic, lactic, butyric, pyruvic, tartaric, succinic, fumaric, L-malic, itaconic and citric, HA) extracted with tri-n-octylamine (TOA) and di-2-ethylhexylphosphoric acid (D2EHPA, HR) were measured. Synergism was observed in the extraction of all acids investigated when a mixed extractant of TOA and D2EHPA was used, while the extraction ability of TOA or D2EHPA alone was negligible under the experimental conditions. In the synergistic system, the distribution ratio of the acids plotted against the D2EHPA concentration showed a bell shape. FT-IR spectrum of the mixed extractant suggested that TOA and D2EHPA interacted. From these results, we estimated that TOA(.)HR and TOA(.)(HR)(2) Were the active and inactive species for the acid extraction, respectively, and that the extracted species was HA(.)TOA(.)HR. The calculated distribution ratios of organic acids showed good agreement with the experimental results. The obtained synergistic extraction equilibrium constants roughly correlated with the hydrophobicity of each acid.