A novel reagentless biosensor for the square wave voltammetric determination of hydroquinone was developed by immobilization of gilo (Solanum gilo) crude extract in the chitosan biopolymer. The gilo tissue was obtained as a source of peroxidase and immobilized on a chitosan matrix using four different methods: physical adsorption, carbodiimide, glutaraldehyde and carbodiimide/glutaraldehyde. The highest biosensor performance was obtained after the immobilization of peroxidase in chitosan by activation of hydroxyl groups with carbodiimide and cross-linking with glutaraldehyde, which was incorporated in a carbon paste electrode. In the presence of hydrogen peroxide this enzyme catalyses the oxidation of hydroquinone to quinone and the silver/silver chloride (3.0 M KCl). The recovery of hydroquinone from the samples ranged from 95.0 to 105% and a rectilinear analytical curve for hydroquinone concentration from 0.25 to 5.5 mM (r = 0.9996) was obtained. The detection limit was. 0.002 mM and the relative standard deviation was lower than 1.0% for 0.3 mM hydroquinone in 0.1 M phosphate buffer solution at pH 7.0 (n = 8). The lifetime of this biosensor was 6 months (at least 300 determinations). (c) 2005 Elsevier Inc. Alt rights reserved.