The substrate specificity of sheep liver sorbitol dehydrogenase has been studied by steady-state kinetics over the range pH 7-10. Sorbitol dehydrogenase stereo-selectively catalyses the reversible NAD-linked oxidation of various polyols and other secondary alcohols into their corresponding ketones. The kinetic constants are given for various novel polyol substrates, including -glucitol, -mannitol, -altritol, -altritol, -iditol and eight heptitols, as well as for many aliphatic and aromatic alcohols. The maximum velocities (k cat ) and the substrate specificity-constants (k cat \K m ) are positively correlated with increasing pH. The enzymecatalysed reactions occur by a compulsory ordered kinetic mechanism with the coenzyme as the first, or leading, substrate. With many substrates, the rate-limiting step for the overall reaction is the enzyme-NADH product dissociation. However, with several substrates there is a transition to a mechanism with partial rate-limitation at the ternary complex level, especially at