A sensor able to evaluate the antioxidant activity, has been developed using adenosine and guanosine nucleosides immobilized on carbon electrodes. The experimental variables were optimized for both nucleosides and two carbon electrodes (glassy carbon, GCE, and carbon paste, CPE), showing small differences due mainly to the surface of the CPE that is greater than the CGE. The effect of the simultaneous immobilization of both nucleosides was also studied. The sensor was validated using Fenton reagent in the absence and presence of ascorbic acid as antioxidant. It was observed that the antioxidant protects the nucleoside immobilized on the electrode surface from the oxidizing action of Fenton\'s reagent.