The experimental results of yeast-lipase extracted by AOT-reverse-micelles in isooctane are reported. The effects of pH values and ionic strength in the aqueous phase, surfactant concentration, phase volume ratio, temperature, stirring time, and cosolvent concentration on the phase transfer of the lipase by the reverse micelles are studied. One hundred percent of forward extraction yield, 68% of back extraction yield, and 45% of activity recovery yield for the lipase are obtained. The phase transfer of the lipase is controlled by electrostatic and hydrophobic interactions. (C) 2003 Elsevier Inc. All rights reserved.