Horseradish peroxidase (HRP) was immobilized on carboxylated multi-wall carbon nanotubes in the presence of a coupling reagent, 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide. The immobilized HRP maintained its oxidative activity for guaiacol over a broad range of pH values (4–9). An electrode of graphite rod, 6 mm diam. was fabricated using the immobilized HRP. Cyclic voltammetry of the enzyme electrode confirmed electron transfer between the immobilized HRP and the electrode in the presence of H2O2 but without an added mediator or a reducing substrate.