The present study evaluated the individual and synergistic effects of eight lignocellulose-derived inhibitors on biohydrogen fermentation and their inhibitory mechanisms. The growth of Thermoanaerobacterium thermosaccharolyticum MJ1 (T. thermosaccharolyticum MJ1) was completely inhibited by 1.5 g/L ferulic acid and 0.5 g/L p-coumaric acid, while other inhibitors (<= 2 g/L) exhibited the weaker inhibition. The inhibition on biohydrogen by vanillin and syringaldehyde was much stronger than growth, which was different with other inhibitors. At 2 g/L vanillin and syringaldehyde, the relative maximum OD600 values were 64.7% and 78.2% respectively, however the relative biohydrogen productions were only 12.6% and 33.6% respectively. The lag phase of biohydrogen production was significantly prolonged by 0.3 g/L p-coumaric acid (7.5 times higher than control). Metabolite analysis showed that the metabolic flux had been redirected by vanillin and syringaldehyde. The increased lactate production and decreased production of acetate and butyrate contributed to a lower biohydrogen production. The synergistic effects of inhibitors on growth and biohydrogen production were studied. The main functional inhibitors in mixture were vanillin and syringaldehyde, which were further demonstrated by metabolite analysis. The present work provided a comprehensive insight of inhibitors on biohydrogen fermentation. (C) 2019 Elsevier Ltd. All rights reserved.