A mathematical model for several working conditions was numerically solved to study a fixed bed reactor with a biocatalyst. The effect of bed swelling, mass transfer limitations, axial dispersion, residence times and inlet lactose concentration were studied. A Michaelis-Menten kinetics with competitive product inhibition was used to represent the enzymatic reaction of the lactose hydrolysis by the beta-galactosidase. Lactose conversion was used to determine the appropriate conditions for the immobilized enzyme reactor. The model predicted the experimental data with errors less than 2.5%. (c) 2006 Elsevier Ltd. All rights reserved.