A comparison is made of potentiometric and amperometric sensors. For amperometric sensors there are advantages in using the wall–jet system with ring–disc or packed-bed electrodes. Particular applications to the determination of proteins, root death, total iron and the concentration of NO3– are described. Enzyme electrodes using organic salts are capable of the direct oxidation of the enzyme itself. A theoretical description is presented. Experiments with glucose oxidase show that the transport of glucose through the membrane is the rate-limiting step. By contrast with a sensor for choline, using choline oxidase, we find that the transport of substrate through the membrane and the unsaturated enzyme kinetics are each partially rate-limiting. Electrodes, made of conducting organic salts, also oxidise NADH. For an ethanol sensor, using ethanol dehydrogenase, we find that the rate-limiting steps are those involving homogeneous enzyme kinetics.