A SiO2/Nb2O5 mixed oxide was prepared by a sol-gel processing method based on TEOS and NbCl5 as precursors and HCl as catalysts. A material having a specific surface area of 703 m2 g-1, average pore diameter of 2.4 nm and 5 wt.% of Nb was obtained. An amperometric peroxidase-based biosensor for phenol was constructed by immobilizing the enzyme onto the SiO2/Nb2O5 sol-gel matrix by adsorption and cross-linking with glutaraldehyde and mixing with graphite powder to make a modified carbon paste. The biosensor performance for phenol detection, investigated in a flow injection system, was based on mediated electron transfer of horseradish peroxidase (HRP), avoiding the direct electron transfer of HRP, which was blocked by the sol-gel matrix. With optimized conditions, a linear response range from 5 to 25 μmol dm-3 for phenol was obtained with a sensitivity of 3.2 nA dm3 μmol-1. The detection limit of the biosensor for phenol was 0.5 μmol dm-3 and the analytical frequency was 27 samples h-1. The biosensor response was tested for various phenol substrates and the highest response was observed for 2-amino-4-chlorophenol. During 200 determinations, the biosensor kept the same response for phenol. The modified carbon paste retained its activity during 6 months of storage under refrigeration. © 2002 Elsevier Science Ltd. All rights reserved.