A simple thermodynamic model is developed for the extraction of proteins with reverse micelles formed with the contact method. The model is based on the ion-exchange reaction of the protein and the surfactant counterion at the reverse micellar interface. Using the equilibrium constant for this reaction and the equilibrium constants of the protein reactions in the aqueous phase, a simple expression is derived for the effects of salt type and concentration, pH, surfactant concentration, and volume ratio of two phases on the extraction. Results on the extraction of a-chymotrypsin into dioctyldimethyl ammonium chloride (DODMAC) reverse micelles are well predicted by the model. The negatively charged proteins are extracted from the aqueous phase by exchanging with the Cl- counterion of DODMAC at the reverse micellar interface. The presence of counterions different from chloride in the system, which are introduced through addition of a salt, has a significant effect on the extraction. The added counterions are exchanged with the chloride of the surfactant at the reverse micellar interface, therefore changing the nature of the surfactant. This change in the nature of the surfactant, in rum, alters the extraction of negatively charged proteins.