The kinetic behaviour of beta-galactosidase from Kluyveromices marxianus (Saccharomyces) lactis, immobilized on different oxides supports, such as alumina, silica, and silicated alumina has been studied. We observed a strong dependence of the immobilized enzyme activities on the chemical nature and physical structure of the supports. In particular, when the particle sizes of the supports are increased, the enzymatic activity strongly decreases. The hydrolysis of lactose, promoted by the mentioned enzyme immobilized on small commercial silica spheres, has been studied in different operative conditions, by changing: feed rate, reagent and products concentration and temperature, while pH has been kept constant (7.0). A depressing effect originated by both the reaction products, has been observed. Therefore, a kinetic model based on the Michaelis-Menten mechanism, in which the depressing effects of both the hydrolysis products (galactose and glucose) are also considered, have been developed and the related parameters determined. (C) 2003 Elsevier Science B.V. All rights reserved.