Anaerobic production of hydrogen from palm oil mill effluent (POME) by microflora. was investigated in 5-1 bioreactor at 60 degrees C and pH 5.5. POME sludge was collected from the anaerobic pond of a POME treatment plant at a palm oil mill and used as a source of inocula. A batch reactor was found to yield a total of 4708 ml H-2/(l POME) and the maximum evolution rate was 454 ml-H-2/(l POME h). A fed batch process was conducted after 50 h. Two liters of reaction medium was removed and 21 of fresh POME was added to the reactor every 24 h (15 times). The reproducibility of the fed batch process checked by changing the feeding time every 8 h (10 times). A yield of 2382 ml H-2/(l POME) and 2419 ml H-2/(l POME) at maximum evolution rate of 313 ml H-2/(l POME h) and 436 ml H-2/(l POME h) were obtained, respectively. Throughout the study, methane gas was not observed in the evolved gas mixture. (c) 2004 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.