In this study, the optimal fermentation type and the operating conditions of anaerobic process in continuous-flow acidogenic reactors was investigated for the maximization of bio-hydrogen production using mixed cultures. Butyric acid type fermentation occurred at pH > 6, propionic acid type fermentation occurred at pH about 5.5 with E-h (redox potential) >-278 mV, and ethanol-type fermentation occurred at pH < 4.5. The representative strains of these fermentations were Clostridium sp., Propionibacterium sp. and Bacteriodes sp., respectively. Ethanol fermentation was optimal type by comparing the operating stabilities and hydrogen production capacities between the fermentation types, which remained stable when the organic loading rate (OLR) reached the highest OLR at 86.1 kgCOD/m(3) d. The maximum hydrogen production reached up to 14.99 L/d. (c) 2006 Elsevier Ltd. All rights reserved.