Urease was immobilized onto silica gel and vulcasil activated with Ti4+ and V5+. Higher activity was observed for urease immobilized onto vulcasil compared to that bound to silica gel. The carriers activated with Ti4+ showed activity higher than that of carriers activated with V5+. Activity was affected also by the temperature at which the modified carrier was dried. Lower activities were measured with the carriers heated to 550 degrees C. The highest activity showed urease metallochelate-bound to vulcasil activated with Ti4+ and dried at 110 degrees C and the values were close to these of the free enzyme. Thermal stability, pH(opt), T-opt and kinetic parameters of the immobilized urease were studied. The bound enzyme preserved its activity in wider pH and Tintervals and showed higher thermal stability compared to the native one. The highest V-max of the enzyme reaction was measured for urease metallochelate-bound to vulcasil activated with Ti4+, (dried at 110 degrees C) and the values were close to these of the free enzyme. The activation energies of the enzymes bound to all types of carriers was ca. 4% smaller than that of the free enzyme. (c) 2005 Elsevier Ltd. All rights reserved.