The extraction equilibria of six kinds of amino acids with di (2-ethylhexyl) phosphoric acid (D 2 EHPA) dissolved in n-heptane were studied at 298.2 K in the low pH range (2 similar to 5). The distribution ratios of amino acid D+ as the cationic species were inversely proportional to the Ist power of the concentration of hydrogen ion, and directly to the 0.5 th power of the concentration of D 2 EHPA. The extraction equilibrium constants K-ex of the amino acids used were 0.650 (Phe), 0.420 (Trp), 0.320 (Leu), 0.275 (Met), 0.064 (Gly) and 0.040 (Ala) mol(?)(0.5) . m(-L5). The highest K-ex value was obtained for Phe, being larger than the value for Ala by a factor of about 16. It was found that the extraction equilibrium constants of amino acids increase with increasing hydrophobicity.