Continuous biohydrogen production by acclimatized anaerobic sludge was investigated using glucose and xylose individually at a concentration of 10 g/L and furfural concentrations of (0, 0.25, 0.5, 1, 2, and 4 g/L). The glucose-fed reactor showed that the initial hydrogen yield of 2.27 mol H-2/mol glucose increased by 17% and 6% at furfural concentrations of 0.25 and 0.5 g/L, respectively, and decreased by 21%, 29% and 62% at furfural concentrations of 1, 2, and 4 g/L, respectively. The inhibition threshold for furfural was in the range of 2-4 g/L. The revivability of the inhibited sludge was confirmed by eliminating furfural addition, which resulted in a hydrogen yield of 1.64 mol H-2/mol glucose comparable to the 2.27 mol H2/mol glucose observed initially without furfural. A similar trend was observed in the xylosefed reactor, in which the hydrogen yield decreased by 63% at the 4 g/L furfural to 0.57 mol H-2/mol xylose. (C) 2016 Elsevier Ltd. All rights reserved.