The authors explored the inhibitory effect of 5-hydroxymethylfurfural (5-HMF) on continuous hydrogen fermentation using a mixed microbial population. A lab-scale fixed bed reactor (FBR) with a 3.0-L working volume was fed with 15 g/L of glucose as sole carbon source at 6 h HRT, while the 5-HMF concentration in feedstock was increased stepwise from 1.2 to 2.7 g/L. 5-HMF gradually decreased hydrogen production. At 2.7 g 5-HMF/L, hydrogen yield, and hydrogen production rate were 0.86 0.08 mol H-2/mol glucoseadded and 7.4 L H-2/L-d, respectively, which were 15% of the yield and production rate observed without 5-HMF. Quantitative polymerase chain reaction (qPCR) analysis showed that 5HMF changed the dominant bacteria from Clostridium sp. to Lactobacillus sp. Scanning electronic microscope (SEM) showed the biofilm structure of mixed culture was also damaged by 5-HMF. However, the inhibition was milder than that on batch fermentation reported earlier. Furthermore, the H-2-producing performance, bacterial population, and granular morphology were partially recovered when 5-HMF was removed. The resistance to 5-HMF inhibition would result from the acclimation during continuous operation and attached growth microorganism by immobilization and entrapment. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.