Batch cultures of Clostridium acetobutylicum at controlled pH values of 5 and 5.5 were carried out in a three-electrode potentiometric system with methyl viologen (1 mM) as electron carrier. Although an irreversible loss of methyl viologen at the electrode surface was observed, a significant increase in alcohol yield was obtained. In comparison to control fermentation with or without methyl viologen addition; the butanol yield improvements were respectively of 7 or 51% at pH 5, and 56 or 467% at pH 5.5.