Active membranes prepared by covalent attachment of Candida antarctica lipase B (CALB) onto a ceramic Support previously coated by polymers (gelatine/polyethyleneimine PEI) were tested in a membrane reactor under different hydrodynamic conditions (Reynolds number and transmembrane pressure) for the synthesis of butyl laurate in hexane. The ester production was found to be directly related to the permeation flux across the membrane. However, the permeate flux unexpectedly decreased when the Reynolds number values increased. Scanning electron microscopy (SEM) and mercury porosimetry showed that this phenomenon is related to morphological changes which occurred in the polymer layer when the tangential velocity varied. (C) 2007 Elsevier B.V. All rights reserved.