Thermal deactivation of free and immobilized beta-glucosidase from Penicillium funiculosum has been studied in the temperature range 30-70 degrees C. Immobilization results in a remarkable increase in beta-glucosidase thermal stability. A two-step series deactivation mechanism has been used to describe the experimental results for both free and immobilized beta-glucosidases. The kinetic equations resulting from this model fit our experimental results with a mean deviation of less than 10%.