The extremely thermostable beta-glycosidase from Pyrococcus furiosus was used for the production of oligosaccharides with lactose as a substrate. Using a thermozyme made it possible to operate at higher reaction temperatures, and thus to increase the substrate concentration. This increased the substrate concentration and the subsequent lower water concentration suppressed hydrolysis and therefore improved the oligosaccharide yield. During the reaction, brown pigments were formed, caused by Maillard reactions. This changes the structure of the enzyme and causes faster inactivation of the enzyme, compared to normal inactivation by temperature. This faster inactivation is the main design criterion for the reaction system. Reduction of Maillard reactions can be done by altering the process conditions or through modification of the enzyme, either chemically or by altering the enzyme structure through genetic modifications. In this work, chemical modification of the enzyme was chosen by covalent immobilisation on Eupergit. Unfortunately, the immobilisation did not reduce Maillard reactivity. (C) 2002 Published by Elsevier Science B.V.