Thermoanaerobacterium-rich sludge was used for hydrogen production and phenol removal from palm oil mill effluent (POME) in the presence of phenol concentration of 100 similar to 1000 mg/L. Thermoanaerobacterium-rich sludge yielded the most hydrogen of 4.2 L H-2/L-POME with 65% phenol removal efficiency at 400 mg/L phenol. Butyric acid and acetic acid were the main metabolites. The effects of oil palm ash, NH4NO3 and iron concentration (Fe2+) on hydrogen production and phenol removal efficiency from POME by Thermoanaerobacterium-rich sludge was investigated using response surface methodology (RSM). The RSM results indicated that the presence of 0.2 g Fe2+/L, 0.3 g/L NH4NO3 and 20 g/L oil palm ash in POME could improved phenol removal efficiency, with predicted hydrogen production and phenol removal efficiency of 3.45 L H-2/L-POME and 93%, respectively. In a confirmation experiment under optimized conditions highly reproducible results were obtained, with hydrogen production and phenol removal efficiency of 3.43 +/- 0.12 L H-2/L-POME and 92 +/- 1.5%, respectively. Simultaneous hydrogen production and phenol removal efficiency in continuous stirred tank reactor at hydraulic retention time (HRT) of 1 and 2 days were 4.0 L H-2/L-POME with 85% and 4.2 L H-2/L-POME with 92%, respectively. Phenol degrading Thermoanaerobacterium-rich sludge comprised of Thermoanaerobacterium thermosaccharolyticum, Thermoanaerobacterium aciditolerans, Desulfotomaculum sp., Bacillus coagulans and Clostridium uzonii. Phenol degrading Thermoanaerobacterium-rich sludge has great potential to harvest hydrogen from phenol-containing wastewater. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.