Microbial fuel cells (MFCs) were used to monitor metabolism changes in Clostridium acetobutylicum fermentations. When MFCs were inoculated with C. acetobutylicum, they generated a unique voltage output pattern where two distinct voltage peaks occurred over a weeklong period. This result was markedly different to previously studied organisms which usually generate one sustained voltage peak. Analysis of the fermentation products indicated that the dual voltage peaks correlated with glucose metabolism. The first voltage peak correlated with acidogenic metabolism (acetate and butyrate production) and the second peak with solventogenic metabolism (acetone and butanol production). This demonstrates that MFCs can be applied as a novel tool to monitor the shift from acid production to solvent production in C. acetobutylicum. Published by Elsevier Ltd.