Using vacancy solution theory based on a surface pressure, a thermodynamic model has been developed for prediction of the protein partitioning in aqueous solution in equilibrium with an oil phase containing reverse micelles. In the present model, one vacancy solution represents the bulk aqueous phase and the other the reverse-micellar phase. Using the concentration of protein in both reverse micelles and aqueous phase, the surface pressure was incorporated for prediction of the protein adsorption in reverse micelles and the nonideality of the system was expressed in terms of the Wilson activity-coefficient model. The present model has been applied to the extraction of bovine serum albumin with formation of reverse micelles using a cationic surfactant, cetyltrimethylammonium bromide (CTAB), in isooctane-l-hexanol. The results of prediction were in very good agreement with the experiment.