AbstractGrowing scientific interest in mixed microbial culture‐based anaerobic biotechnologies for the production of value‐added chemicals and fuels from organic waste residues requires a parallel focus on the development and implementation of strategies to control the distribution of products. This study examined the feasibility of an electrofermentation approach, based on the introduction of a polarized (−700 mV vs. the standard hydrogen electrode) graphite electrode in the fermentation medium, to steer the product distribution during the conversion of organic substrates (glucose, ethanol, and acetate supplied as single compounds or in mixtures) by undefined mixed microbial cultures. In batch experiments, the polarized electrode triggered a nearly 20‐fold increase (relative to open circuit controls) in the yield of isobutyrate production (0.43±0.01 vs. 0.02±0.02 mol mol−1 glucose) during the anaerobic fermentation of the ternary mixture of substrates, without adversely affecting the rate of substrate bioconversion. The observed change in the fermentative metabolism was most likely triggered by the (potentiostatic) regulation of the oxidation–reduction potential of the reaction medium rather than by the electrode serving as an electron donor.