Deactivation studies of jack bean urease immobilized on porous alumina beads in the hydrolysis of urea were conducted in a continuously stirred tank reactor (CSTR) at a temperature of 25 degrees C and pH 7.0. Though the mechanism of poisoning of urease by product ammonia is fairly well understood from the literature, the nature of the poisoning of urease by urea is presented in this article. These studies were conducted by adsorbing the ammonia formed in the hydrolysis reaction. The results indicate that, in the presence of the adsorbent Zeolite W, the deactivation rate is reduced by a factor of almost two, and thus provide a technique for prolonging the life of the enzyme. The deactivation model suggests that the free form of the enzyme is most susceptible to attack by the substrate urea. The experimental data suggest that deactivation by combined ammonia and urea is fairly complex.