The asymmetric hydrolyzation of racemic ibuprofen ester catalyzed by lipase is one of the most important methods for the chiral separation of ibuprofen. In this work, a special microstructure in the composite hydrophilic cellulose acetate (CA)/hydrophobic polytetrafluoroethylene (PTFE) membrane was designed for lipase immobilization by ultrafiltration. A biphasic enzymatic membrane reactor (EMR) and an emulsion reaction system with free lipase were both used, and the activity, enantioselectivity selectivity and half-life of immobilized and free enzymes were compared. The morphology of the composite membrane and the position of the entrapment in the microstructure of composite membrane where lipase were immobilized were observed by scanning electron microscopy (SEM). The effects of substrate concentration, enzyme loading, reaction temperature and pH on the separation were investigated. The experimental results showed that the lipase was entrapped at the interface of the composite membrane, which is consistent with the interface of aqueous phase and organic phase. Because the immobilized enzyme by our new method did not destroy the enzyme structure, it retained higher activity; the enzyme activity was more than 60% compared to the free lipase when enzyme loading was under 1 g-protein/m(2). Meanwhile, the immobilized enzyme provided better chiral selectivity (up to 83.5% ee(p)) and a longer half-life (about 183.3 h in on c work compared to 94 It as reported by other researcher). High enzyme activity and chiral selectivity were obtained with a substrate concentration of 0.35 M, enzyme loading of 0.7-1.0 g-protein/m(2) at aqueous phase pH 8.0 and a temperature of 40 degrees C. (c) 2007 Elsevier B.V.. All rights reserved.