Covalent immobilization of C. antarctica lipase B (CALB) on sepharose, alumina, and silica was undertaken. The thermal stability of these covalently immobilized catalysts were studied and compared to adsorbed derivatives from Novo Nordisk at 50 degrees C under wet conditions. Native enzyme and Novozym 435 follow a deactivation model E --> E-1 whereas covalently immobilized derivatives and SP435A follow the model E --> E-1 --> E-2. This different behavior is related to the nature of the support and the immobilization methodology. Water absorption isotherms of dry solid biocatalysts in air or isooctane were used to predict the optimum preequilibrium a(w) value to obtain the highest rate in the esterification of (R,S)-ibuprofen.