The effect of FeSO4 on continuous H-2 production in a membrane bioreactor (MBR) was investigated using anaerobic mixed microflora under mesophilic condition. The H-2 production of 41.6 l/day was obtained at 10.9 mg FeSO4/l, which was 1.59 times higher than that at 2.7 mg FeSO4/l. Between 2.7 and 13.7 mg FeSO4/l, the H-2 production rate increased in parallel with the H-2 yield under high-cell-density condition. For the same amounts of FeSO4, increases in butyric acid together with decreases in lactic acid promoted a reduction of the number of protons and the resultant release of H-2. The hydrogenase activity of 1.08 mg methylene blue (M.B) reduced/min at 10.9 mg FeSO4/l was about sixfold higher than at 2.7 mg FeSO4/l. These results suggest that the addition of iron and sulfur to an MBR is an important key factor in the enhancement of H-2 production. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.