A comparative study was made on the extraction equilibria of succinic and tartaric acids from aqueous solutions with tri-n-octylamine in xylene. It was shown that the extractability of tartaric acid was higher than succinic acid under comparable conditions. The compositions of the multiple-extracted complexes in the organic phase and the equilibrium constants for the formation of the complexes were numerically determined. Furthermore, the mole fractions of the species present in the organic phase were quantitatively obtained as a function of initial concentrations of the acids and amine. The effect of temperature on the extraction of acids was also examined and a change of complex composition with temperature was seen for succinic acid in the temperature range 293-323 K. Finally, the related thermodynamic data were calculated.