Cationic surfactants normally need an alcohol as cosurfactant to form reverse micelles. A thermodynamic model has been developed to predict the concentrations of all ionic species, including the cationic surfactant, in the aqueous and the reverse micellar phases as a function of the concentration of cationic surfactant and of cosurfactant for the extraction of amino acids by reverse micelles. The model has been tested by performing flash calculations for the system of DODMAC, 1-decanol, isooctane, NaCl, and water, containing three different amino acids. The calculated results are in good agreement with the experimental data.