A diffusion-reaction model suitable for high enzyme loading density was developed to describe the situation of immobilized lipase in the hydrophilic cellulose acetate (CA)/hydrophobic polytetrafluoroethylene (PTFE) composite membrane. Under this situation, not all the immobilized enzyme was found to be effective, and as such, a utilization coefficient of immobilized lipase was proposed. In this paper, two different reaction systems (racemic ibuprofen ester chiral separation and olive oil hydrolysis) using immobilized lipase in hydrophilic/hydrophobic composite membrane was studied. The former system is reaction controlled and the latter is diffusion controlled. The utilization coefficient of lipase catalyzing the reacemic ibuprofen ester was from 83% to 39%, decreasing with the increasing of enzyme loading from 0.3 to 2.1 g m(-2). The calculation values were found to be consistent with the experimental data through the utilization coefficient of lipase, which was obtained by the first system to predict the reaction rate of the second one. The effect of substrate concentration and membrane structure on the activity of the immobilized lipase was investigated with this model. We conclude that (1) the effect of substrate concentration is double-edged, (2) utilization coefficient of lipase decreases with the increasing of enzyme loading, (3) composite membranes with thinner and more porosity hydrophobic layer leads to higher reaction rate. (c) 2007 Elsevier B.V. All rights reserved.