An electrochemical bioreactor with glucose dehydrogenase immobilized on to the electrode surface produced gluconic acid from glucose with concomitant recycling of the NAD(+) coenzyme at 0.7 V. Since the enzyme is deactivated during operation at this redox potential, co-immobilization of 3, 4-dihydroxybenzaldehyde as mediator allowed the system to operate at 0.2 V and increased both the activity (2.4-times) and the stability of the immobilized enzyme (2.2-times). The different effective electrochemical surfaces resulting from the different mediator immobilization modes are important in determining these three properties.