Abstract An amperometric sensor for glucose oxidase is described. Partial optimization of the working electrode potential at 0·4 V vs. Ag/AgCl, 0·1 M KCl at a glucose concentration of 30 mM allows a low detection limit of 50 pM glucose oxidase within 25 min. The sensor exhibits a high degree of precision and reproducibility, and the same electrode may be re-used for many successive quantitative determinations. A mechanism for the sensor is proposed, in which hydrogen peroxide generated by glucose oxidase in bulk solution is oxidized at the working electrode, generating a current response that can be related to the glucose oxidase concentration. Conjugation of glucose oxidase with a coumarin-based pesticide analogue does not affect the amperometric response.