A kinetic model is presented for the operation of an amperometric enzyme electrode in which a single electroactive enzyme oxidizes both the initial substrate and the primary reaction product. Two examples of this type of electrode have been investigated : the hypoxanthine + xanthine and choline + betaine aldehyde systems, which utilize the enzymes xanthine oxidase and choline oxidase respectively. The conducting organic salt tetrathiafulvalinium tetracyanoquinodimethanide is used as the electrode material in both sensors.