This paper describes the development of a xanthine oxidase/poly-m-phenylenediamine (XOD-PPD)-modified electrode. The biosensor was constructed by encapsulating XOD in a sol-gel matrix deposited onto a platinum based screen-printed electrode functionalized with a permselective PPD membrane. The hydrogen peroxide generated as a final product of the enzymatic reaction between the hypoxanthine and the XOD or by the spontaneous dismutation of superoxide radicals was selectively monitored at +700 mV. The use of a highly selective PPD layer blocked the nonspecific oxidation of other oxidizable molecules. Finally the biosensor was applied to the determination of the antioxidant capacity of acetylsalicylic acid.