Ion strength is one of the important conditions to control extraction of lysozyme with AOT reversed micelles. The extraction equilibrium changes largely with a type of the salt, i.e. cation species. For the three salt systems, KCl, BaCl2 and CaCl2, extracted fraction of lysozyme is measured for various pH values. For divalent cation systems, the behavior of the protein-extracted fraction against pH is quite different from that for KCl system. The salt concentration has a strong influence on the extracted fraction and the characteristic trend is explained by the manner of the interaction between divalent cation and anionic surfactant, AOT. Difference of the trend in the extracted fraction between barium and calcium systems is also discussed in terms of hydration of the cations. Because of the larger hydration number, combination of calcium ion to interfacial layer of AOT makes the interface rather flexible, thus the cation exchange occurs more frequently than in barium system. (C) 2003 Elsevier B.V. All rights reserved.