Dark fermentation using mixed cultures is an attractive biological process for producing hydrogen (H-2) from lignocellulosic biomass at a low cost. Physicochemical pretreatment is generally used to convert lignocellulosic materials into monosaccharides. However, the processes also involved release degradation byproducts which can, in turn, inhibit microbial growth and metabolism and, hence, impact substrate conversion. In this study, the impact on H-2 production of lignocellulose-derived compounds (i.e. furan derivatives, phenolic compounds and lignins) was assessed along with their effect on bacterial communities and metabolisms. Batch tests were carried out using xylose as model substrate (1.67 mol(H2) mol(xylose)(-1) in the control test). All the putative inhibitory compounds showed a significant negative impact on H-2 production performance (ranging from 0.34 to 1.39 mol(H2) mol(xylose)(-1)). The H-2 yields were impacted more strongly by furan derivatives (0.40-0.51 mol(H2) mol(xylose)(-1)) than by phenolic compounds (1.28-1.39 mol(H2) mol(xylose)(-1)). Except for the batch tests supplemented with lignins, the lag phase was shorter for inhibitors having the highest molecular weight (8 days versus 22 days for the lowest MW). Variability of the lag phase was clearly related to a shift in bacterial community structure, as shown by multivariate ordination statistics. The decrease in H-2 yield was associated with a decrease in the relative abundance of several H-2-producing clostridial species. Interestingly, Clostridium beijerinkii was found to be more resistant to the inhibitors, making this bacterium an ideal candidate for H-2 production from hydrolyzates of lignocellulosic biomass. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.