In this study, zero valent iron (ZVI) was applied to enhance hydrogen fermentation (H-2). Glucose was used as a substrate at various concentrations of 5-40 g COD/L, and ZVI concentration was adjusted at 0-10 g/L. Without ZVI addition, H-2 yield decreased as substrate concentration increased, due to organic acids accumulation as substrate concentration increased. At low substrate concentration of 5 g COD/L, there was a slight increase of H-2 yield from 1.4 to 1.5 mol H-2/mol glucose added by ZVI addition. However, the increment was substantial at higher substrate range. For example, H-2 yield increased from 0.9 to 1.2-1.5 mol H-2/mol hexoseadded by ZVI addition at 20 g COD/L. The results indicated that ZVI may create a more favorable environment for anaerobic microbial acidogenesis with providing buffering effect to prevent pH drop due to produced hydroxyl ions as it dissolved. These changes of environmental conditions also brought differences on organic acids profile and microbial community, and those may affect to overall H-2 fermentation performances. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.