Past research has suggested that furfural and hydroxymethylfurfural (HMF) present in lignocellulosic hydrolyzates exert a synergistic effect on dark fermentative H-2 production, which has not adequately proven in microbial consortia. It is possible that some members of a consortium experience less inhibition than others, helping to the entire consortium to overcome the inhibition. To elucidate the type of inhibitory effect that these agents exert, the objective of this study was to contrast the individual impacts of furfural and HMF with the corresponding mixtures at the same concentration threshold on the hydrogen production and the relative abundance of different members in the consortium. Heat-treated anaerobic granules served as inoculum to ferment xylose for hydrogen production in presence of furfural (0.10, 0.50, and 1.00 g/L), HMF (0.02, 0.09, 0.19 g/L) and the corresponding mixtures in comparison with a control in absence of these agents. Furfural alone did not inhibit the hydrogen production; indeed, the inoculum completely degraded furfural at all its concentrations with the presence of furoic acid. HFM was partially degraded in the treatments with the lowest/middle concentrations, resulting in higher hydrogen production than the control. In contrast, at the highest HMF concentration, the inoculum was unable to remove it resulting in the strongest inhibition of hydrogen production. All the F/HMF mixtures had an inhibitory effect on the hydrogen production. A log2 fold change analysis of pyrosequencing reads evidenced that these agents promoted the growth of Lactobacillus, and clostridia species such as Clostridium butyricum, Clostridium bifermentans, and Clostridium sartagoforme, suggesting their active participation in the detoxification process. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.