The conversion of fumaric acid into L-malic acid by fumarase immobilized in a membrane reactor was analyzed experimentally. The enzyme was entrapped in asymmetric capillary membranes made of polysulfone. The performance of the reactor was evaluated in terms of conversion degree, reaction rate, and stability. The influence of operating conditions, such as amount of immobilized enzyme, substrate concentration, residence time, and axial flow rate, were investigated. The kinetic parameters K-m, V-max, and k(+2) were also measured. The stability of the immobilized enzyme was very good, showing no activity decay during more than 2 weeks of continuous operation. (C) 2001 John Wiley & Sons, Inc.