Currently, syngas fermentation is being developed as one option towards the production of biofuels from biomass. This process utilizes the acetyl-CoA (Wood-Ljungdahl) metabolic pathway. Along the pathway, CO and CO(2) are used as carbon sources. Electrons required for the metabolic process are generated from H(2) and/or from CO. This study showed that electron production from CO is always more thermodynamically favorable compared to electron production from H(2) and this finding is independent of pH, ionic strength, gas partial pressure, and electron carrier pairs. Additionally, electron production from H(2) may be thermodynamically unfavorable in some experimental conditions. Thus, it is unlikely that H(2) can be utilized for electron production in favor of CO when both species are present. Therefore, CO conversion efficiency will be sacrificed during syngas fermentation since some of the CO will provide electrons at the expense of product and cell mass formation. (C) 2011 Elsevier Ltd. All rights reserved.