In the present work olive mill wastewater (OMW) collected from West Anatolia-Turkey during 2001, containing 36.02 g carbon, 5.26 g hydrogen, and 0.96 g nitrogen in 100 g suspended solid was used as a sole substrate for the production of hydrogen gas by Rhodobacter sphaeroides O.U.001 in 400 ml glass, column-photobioreactors. Hydrogen production studies on diluted-OMW were investigated in the range of 20% (v/v) and 1% (v/v) OMW containing media. Below 5% OMW containing media, bacterial growth rate fitted well to the logistic model where hydrogen production was observed for the ones below 4% OMW. A maximum hydrogen production potential (HPP) of 13.9 I-H2/I-OMW was obtained at 2% OMW. During the biological hydrogen production process, chemical oxygen demand (COD) of the diluted wastewater decreased from 1100 to 720 mg/l; biochemical oxygen demand (BOD) decreased from 475 to 200 mg/l, and the total recoverable phenol content (ortho- and meta-substitutions) decreased from 2.32 to 0.93 mg/l. In addition, valuable by-products such as carotenoid (40 mg/l(OMW)) and polyhydroxybutyrate (PHB) (60 mg/l(OMW)) were obtained. According to these results, OMW was concluded to be a very promising substrate source for biohydrogen production process, with additional benefits of its utilization with regard to environmental and economical aspects. (C) 2003 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.