The separation technique of extraction based on reversible chemical complexation can be a high-capacity and high-selectivity technology for separating polar organic solutes from dilute solution. The Lewis acid, di(2-ethylhexyl)phosphoric acid (D2EHPA), is often chosen as extractant to separate amino acids from aqueous solution because of its high capability in the, reversible chemical complexation. In this paper, trioctylamine (TOA) is used as extractant, and three alpha-amino acids [L-phenylalanine (L-phe), L-tryptophane (L-trp), and L-isoleucine (L-ile)] and aminobenzoic acid (ABC) are used as model amino acids. We present a series. of extraction equilibrium experiments for them with TOA dissolved in n-octane and n-octanol. The effects of initial extractant concentration, equilibrium pH value, and the polarity of bulk solvent on the distribution ratio (D) Of L-phe, L-trp, L-ile, and ABC between the organic and aqueous phases were discussed. Expressions for the distribution ratio are proposed, and the experimental data are in good agreement with the proposed expressions.