The behaviour of an ultrafiltration membrane reactor (UFMR) (60 cm(3) of reactor volume) for the optical resolution of DL-butyrine catalysed by L-aminoacylase was studied, and the influence of substrate concentration (15-25 mmol dm(-3) in N-acetyl-L-butyrine), temperature (30-50degreesC) and the presence of CoCl2 (0.5 mmol dm(-3)) on enzyme deactivation was analysed. Adsorption studies with polysulphone and regenerated cellulose membranes (30 cm(2) of filtration surface), as well as. deactivation studies in the reaction conditions, were carried out to determine the causes of deactivation. A single-step deactivation scheme is proposed, and it was shown that this first-order model adequately describes enzyme deactivation. The dependence of K-d on the enzyme concentration points to the enzyme deactivation, which is mainly caused by the adsorption phenomena on the membrane surface. (C) 2004 Elsevier Inc. All rights reserved.