The physiological response of Clostridium acetobutylicum to methyl and benzyl viologen was investigated. Viologen dyes at low concentrations (at levels of parts per million [micrograms per milliliter]) caused significant metabolic shifts. Altered electron flow appeared to direct carbon flow from acid to alcohol production accompanied by decreased hydrogen evolution. Reducing equivalents normally released as free hydrogen were directed toward formation of NADH which, in turn, resulted in increased alcohol production. In addition, it was shown that solvent production can take place at pH 6.3. Contrary to previous reports, butanol production appears to be independent of high levels of acetate-butyrate and glucose.