A new reaction mechanism to explain the equilibrium behavior of amino-acid extraction using di(2-ethylhexyl)phosphoric acid (D2EHPA) is proposed. This mechanism represents the interaction between amino acid and D2EHPA through a set of two reactions, one at the aqueous–organic interface and the other in the bulk of the organic phase. It quantitatively predicts the hitherto unexplained dependence of apparent equilibrium constant on a single parameter—loading ratio (ratio of concentration of amino acid in the organic phase at equilibrium to initial concentration of carrier). At low loading ratios, reaction in the bulk plays an insignificant role, and interfacial reaction alone can explain the equilibrium behavior. At high loading ratios, both reactions need to be considered. The new mechanism is tested for the extraction of two amino acids, phenylalanine and tryptophan, for which the equilibrium data have been published. New data were also obtained for phenylalanine to test it over a much wider range of parameter values. The proposed mechanism can predict all the data very well.