Hydrogen and carbon dioxide as the main products of anaerobic acetone-butanol-ethanol (ABE) fermentation are very crucial for metabolic flux analysis. Up to the present, rare study has been devoted to real-time analysis on gas products of anaerobic fermentation systems. An on-line off-gas analysis system was therefore designed to monitor the off-gas produced in ABE fermentation with good accuracy. To deal with the influence from oxygen and off-gas variation during the fermentation, a strictly sealed monitoring system was set up to accurately analyze the off-gas while maintaining the oxygen-free environment in the bioreactor. At the meantime, the effect of electron carrier (neutral red) on metabolic distribution was studied in the free cell and immobilized cell fermentations. Compared to 13.7 g/L butanol (21.3 g/L ABE) and 20.8 L H-2 in 44 h in free cell fermentation, 16.8 g/L butanol (26.8 g/L ABE) and 23.9 L H-2 in 35 h in immobilized cell fermentation were produced in the addition of neutral red, suggesting that more NADH was produced for solvents biosynthesis. It is therefore demonstrated that the established on-line off-gas analysis system and optimization strategy for improving butanol production facilitate metabolic analysis and biofuels production in anaerobic ABE fermentation. (C) 2016 Elsevier Ltd. All rights reserved.