We investigated the effect of cations on the protein extraction into AOT/isooctane reversed micellar solutions using alkaline and alkaline-earth metal ions. Cations greatly influenced the extraction ratio of protein in an order of K+ < Rb+ < Cs+ < Na+ < Li+ in monovalent ions and Ba2+ < Sr2+ < Ca2+ in divalent ones. Besides, the extraction ratio is totally higher in divalent ions than in monovalent ones. This effect of ions can be explained by classifying cations into water-structure forming and water-structure breaking ions. The effect of anions is less than that of cations and is in an order of SCN- < Br-< Cl-, consistent with that of lyotropic series. In addition, the extraction ratio is higher for proteins of higher hydrophobicity and it is supposed that the hydrophobicity and stability of the protein-surfactant complex are related to extraction efficiency. (C) 2002 Elsevier Science B.V. All rights reserved.