The aim of this study was to explore the influence of transport properties of membrane on the performance of an enzymatic membrane reactor and to determine the criteria of the membrane selection on basis of a mathematical model of the reactor. In the examined system a reaction producing a weak acid takes place. It was assumed that only the dissociation of this product determines the pH of the reactor. The retentions of the substrate and product were estimated with the use the Kedem-Katchalsky-Spiegler model. The Michaelis-Menten scheme, modified by the incorporation of a relation of the maximal reaction rate to pH, was applied to describe the rate of the reaction. The influence of the reflection coefficients of reagents on the reactor performance has been investigated. The location of regions of non-trivial steady states of practical importance depends significantly on separation capabilities of a membrane. It has been found that the increase of retention of the product increases the range of operational parameters offering non-zero conversion of the substrate. (C) 2010 Elsevier B.V. All rights reserved.