A novel tyrosinase biosensor has been developed for the subnanomolar detection of phenols, based on the immobilization of tyrosinase in a positively charged Al2O3 sol-gel membrane on a glassy carbon electrode. It has been found that Al2O3 sol-gel is perfectly beneficial to the immobilization of tyrosinase, because it not only possesses the general advantages of sol-gel but it also is an effective promoter of the biosensor. The large microscopic surface area, porous morphology, and hydrophilic property of the sol-gel matrix result in high enzyme loading, and the enzyme entrapped in this matrix retains its activity to a large extent. The Al2O3 sol-gel-containing surface also displays an intrinsic electrocatalytic o-quinone response and, hence, offers a high-sensitivity (127 microA mM(-1)) monitoring of phenols. The detection limit is 0.2 nM at a signal-to-noise ratio of 3, the response time is less than 4 s reaching 95% of the steady-state value, and 70% of the activity is retained after 3 months.