Thermal and pH stabilities of free and immobilized alpha-amylase, beta-amylase, and glucoamylase were compared, in which immobilization support was prepared by equal weights of chitosan and activated clay and were cross-linked with glutaraldehyde. It was shown that the relative activities of immobilized enzymes are higher than free enzymes over broader pH and temperature ranges. alpha-Amylase and glucoamylase immobilized on composite bead maintained 81% of their original activities after 50 times of repeated use. Thermal deactivation energies of free and immobilized enzymes were obtained according to the Arrhenius' equation. The Michaelis constant (K-m) and the maximum rate of starch hydrolysis reaction (V-max) were also calculated according to the Lineweaver-Burk plot. It was found that the K-m and V-max values with immobilized enzymes were larger than those with free enzymes, except for the V-max value with glucoamylase. (C) 2004 Elsevier Inc. All rights reserved.