Open tubular carbon electrodes were used for biamperometric measurements of iron (II) in the ferrocyanide/ferricyanide redox couple. This measurement was used to monitor dehydrogenase enzyme reactions by reaction of ferricyanide with reduced nicotinamide-adeninedinucleotide (NADH) in the presence of the enzyme diaphorase. Reagents and samples are mixed in a flowing stream and the electrodes are placed downstream to measure the extent of reaction after a constant time interval. Ethanol was determined by reaction with NAD in the presence of alcohol dehydrogenase (ADH), ferricyanide, and diaphorase. Optimum solution conditions for the above measurements are described.