The construction and performance of an enzyme electrode as an amperometric sensor of xanthine and hypoxanthine is described. Xanthine oxidase has been immobilized in a conductive redox polymer, poly(mercapto-p-benzoquinone), by means of an electropolymerization of mercaptohydroquinone in the presence of the enzyme. An Au-electroplated glassy carbon electrode coated with the resulting polymer film functioned well as a direct response type of sensor, where the polymer chain served as a conductive molecular chain between the active sites in the enzyme and the substrate electrode. Response characteristics as well as kinetic parameters have been evaluated.