Solvent-producing cultures of Clostridium beijerinckii NCIMB 8052 can reduce a variety of aldehydes and ketones to the corresponding alcohols, but the enzymes that catalyse these biotransformations have not been identified. The possibility that butanol dehydrogenases were involved was tested by comparing the ability of solvent- and acid-producing pH- auxostat cultures to reduce representative biotransformation substrates. The ability of the cultures to produce solvents was manipulated by controlling the biomass concentration, and this was achieved by varying the glucose concentration in the inflowing medium. The solvent-producing culture could reduce cyclohexanone and benzaldehyde. In contrast, very little reduction of these substrates occurred in the acid-producing culture. This suggested that one or more butanol dehydrogenases did indeed catalyse these biotransformations.