A mathematical model applicable to the analysis, design and simulation of homogeneous enzymatic processes reacting in a ultrafiltration membrane reactor has been formulated. The model develops the unsteady-state design equations, including a first-order model to represent enzyme deactivation kinetics. To check the model, the stereospecific hydrolysis of N-acetyl-DL-butyrine catalysed by L-aminoacylase to give L-butyrine and unmodified N-acetyl-D-butyrine was used as an experimental system. The experiments were carried out at pH 7.0 and 50 degrees C, using two different flow rates (1.0 and 1.5 cm(3) min(-1)), three different substrate concentrations at the reactor inlet (25, 50 and 100 mmol dm(-3) in N-acetyl-L-butyrine (C-SL0)) and two different start-up modes. When the experimental data were compared to those theoretically predicted, the statistical analysis underlined the goodness of the model and the accuracy of the constants calculated. (c) 2005 Elsevier Ltd. All rights reserved.