The catalytic activity and reaction rate of lipase have been studied using the biocatalyst free in an organic/aqueous emulsion and immobilized in a biphasic organic/aqueous membrane reactor. The first reaction system was realized in a stirred tank reactor. The other was obtained by immobilizing the enzyme in the sponge layer of an asymmetric capillary membrane and recirculating the two phases along the two separate circuits of the membrane module. The performance of the reactors has been studied using two different low water-soluble substrates : triglycerides present in commercial olive oil and (R,S)-cyanomethyl-[2-(4-isobutylphenyl)propionate] (CNE). The effects of substrate viscosity and flow dynamics conditions, such as organic phase flow rate, on the biphasic membrane reactor performance have been evaluated on the basis of observed reaction rate and catalytic activity of free and immobilized lipase for both substrates. It has been observed that free lipase showed higher catalytic activity with olive oil, while immobilized lipase showed higher catalytic activity with CNE which has a lower viscosity than olive oil. The increase of organic phase flow rate negatively affected the reactor performance, with a minor effect when using CNE rather than olive oil. The influence of temperature on the biocatalyst performance with the two substrates has also been investigated. The optimal temperature value of lipase was different for the two substrates : 28 degrees C with CNE and 40 degrees C with olive oil.