The extraction equilibria of propionic, and butyric acids with tri-n-octylphosphineoxide (TOPO) dissolved in seven solvents in different chemical structures {isoamyl alcohol, oleyl alcohol, toluene, methyl tert-butyl ether (MTBE), cyclohexyl acetate, kerosene, isobutyl methyl ketone (MIBK)} have been measured at 298.15 K. The highest distribution coefficient for both of the acids is obtained by TOPO dissolved in MTBE. It was observed that the use of TOPO dissolved in these diluents has increased the distribution coefficients of propionic and butyric acids between organic and aqueous phases up to 51.22 and 12 times, respectively, compared to the use of pure solvents. The extraction efficiencies with TOPO reached the range of (75.21 to 94.42) % for propionic acid and (90.68 to 96.22) % for butyric acid, depending on the solvent type and concentration.