To enhance biohydrogen production from glucose and xylose in the presence of aldehyde inhibitors, reducing agent (i.e., sodium borohydride) was in situ added for effective detoxification. The detoxification efficiencies of furfural (96.7%) and 5-hydroxymethylfurfural (5-HMF, 91.7%) with 30 mM NaBH4 were much higher than those of vanillin (77.3%) and syringaldehyde (69.3%). Biohydrogen fermentation was completely inhibited without detoxification, probably because of the consumption of nicotinamide adenine dinucleotide (NADH) by inhibitors reduction (R-CHO + 2NADH -> R-CH2OH + 2NAD(+)). Addition of 30 mM NaBH4 provided the reducing power necessary for inhibitors reduction (4R-CHO + NaBH4 + 2H(2)O -> 4R-CH2OH + NaBO2). The recovered reducing power in fermentation resulted in 99.3% recovery of the hydrogen yield and 64.6% recovery of peak production rate. Metabolite production and carbon conversion after detoxification significantly increased to 63.7 mM and 81.9%, respectively. (C) 2015 Elsevier Ltd. All rights reserved.