In the membrane phase contactor, the rate of reagent mass transfer from organic phase (a reservoir) to water phase (a reaction phase) depends on the three different resistances: the organic phase resistance, the resistance generated by the membrane and the water phase resistance. In the case when the enzyme is immobilized on the membrane surface and forms a gel layer, an additional mass transfer resistance is generated that could have an important influence on total mass transfer rate. Its value is directly proportional to the layer thickness and assumes particularly high values in the system where the distribution coefficient is much higher than unity. Since the gel layer thickness affects simultaneously the process yield, the decision concerning the amount of protein immobilized on the surface should be considered in detail.