We investigate the behavior of a reaction described by Michaelis-Menten kinetics in an immobilised enzyme reactor (IER). The IER is treated as a well-stirred flow reactor, in which the bound and unbounded enzyme species are immobilized and therefore constrained to remain within the reaction vessel. The product species leaves the bioreactor either in the reactor outflow or by permeating through the semi-permeable reactor wall. We explore how the concentration of recovered product and the reactor productivity vary with process parameters, particularly those associated with the separation of the product from the substrate through the semi-permeable reactor wall. We show that at low residence times membrane extraction through the reactor walls increases the total product concentration recovered whereas at high residence times membrane extraction decreases the total product concentration. We also show that the reactor productivity is maximised at high residence times. For reactor productivity the key control variable is the ratio of the reactor volume to the jacket volume ( * V ). If this ratio is greater than one, then membrane extraction increases the productivity. If this ratio is less than one, then membrane extraction decreases the productivity.