Immobilization of enzymes in a bioreactor often leads to a decrease in the enzyme activity. In this work, some factors that may contribute to this decrease in activity were investigated. Two-phase membrane reactors were used with lipase immobilized on membranes of various thickness. The reactions studied were hydrolysis of triglycerides and peroxidation of fatty acids. The results were compared with data from the literature. It appeared that in the case of hydrolysis, the membrane activities are all of the same order of magnitude (about 20 mu mol m(-2) s(-1)) independent of membrane type or thickness; however, with hydrophilic membranes, the enzyme activities are much higher. A reaction-diffusion model was developed in which two zones in the membrane are discerned-a diffusion layer and a reaction layer. With this model, the thickness of the reaction layer and the degree of enzyme utilization could be estimated. The thickness of the reaction layer was 0.5-7 mu m and the enzyme utilization was 0.2-14% depending on the conditions.