The activity of enzymes is considerably affected by the pH of a reaction mixture. If a product of enzymatic reaction changes pH, as in the case of ester hydrolysis, this causes the non-monotonicity of both substrate and product dependent rate expression. This dependence determines the conversion degree values as well as the multiplicity of steady states in an enzymatic membrane reactor (EMR), i.e. a continuous stirred-tank bioreactor with an enzyme recycle. The transport properties of a membrane used also can modify the static behaviour of the system. The results of multiplicity analysis of enzymatic reaction which produces a weak acid in EMR are presented. To estimate the retention of a product by a membrane, the theory of irreversible thermodynamics by Kedem and Spiegler has been used. On the bifurcation diagrams a trivial steady state (at the conversion degree going to zero) and the non-trivial steady states are shown. Limits of regions of different multiplicity are localised. The effects of product acidity, enzyme properties and transport properties of a membrane on the structure of steady states are discussed. A specially written software in Delphi (TM) has been used for the calculations.