Toxicity prevents the bioenergy content of certain industrial effluents from being recovered. An enriched Clostridium butyricum strain was employed to produce hydrogen by fermentation from cellobiose in the presence of phenol at 200-1500 mgl(-1). The enriched Cl. butyricum yielded the most hydrogen at 2.1 mol H(2) Mol(-1) cellobiose with 600 mgl(-1) phenol. Butyrate was the main metabolite. Cell metabolism was substantially inhibited at a phenol concentration of 1500 mgl(-1). Part of the phenol was co-degraded during the test, helping to eliminate the toxicity of wastewater. Both the pyruvate oxidative decarboxylation pathway and the NADH pathway contributed to biohydrogen production. Phenol toxicity more strongly inhibits soluble hydrogenase than it does membrane-bound hydrogenase. Although the NADH pathway dominated at low phenol concentration, increasing the phenol concentration shifted the biohydrogen pathway toward decarboxylation. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.