1. The kinetics of oxidation of ethanol, propan-1-ol, butan-1-ol and propan-2-ol by NAD(+) and of reduction of acetaldehyde and butyraldehyde by NADH catalysed by yeast alcohol dehydrogenase were studied. 2. Results for the aldehyde-NADH reactions are consistent with a compulsory-order mechanism with the rate-limiting step being the dissociation of the product enzyme-NAD(+) complex. In contrast the results for the alcohol-NAD(+) reactions indicate that some dissociation of coenzyme from the active enzyme-NAD(+)-alcohol ternary complexes must occur and that the mechanism is not strictly compulsory-order. The rate-limiting step in ethanol oxidation is the dissociation of the product enzyme-NADH complex but with the other alcohols it is probably the catalytic interconversion of ternary complexes. 3. The rate constants describing the combination of NAD(+) and NADH with the enzyme and the dissociations of these coenzymes from binary complexes with the enzyme were measured.