Pure culture of Rhodobacter sphaeroides (NRRL- B1727) was used for continuous photofermentation of volatile fatty acids (VFA) present in the dark fermentation effluent of ground wheat starch. The feed contained 1950 +/- 50 mg L(-1) total VFA with some nutrient supplementation. Hydraulic residence time (HRT) was varied between 24 and 120 hours. The highest steady-state daily hydrogen production (55 ml d(-1)) and hydrogen yield (185 ml H(2) g(-1) VFA) were obtained at HRT = 72 hours (3 days). Biomass concentration increased with increasing HRT. Volumetric and specific hydrogen formation rates were also maximum at HRT = 72 h. High extent of TVFA fermentation at HRT = 72 h resulted in high hydrogen gas production. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.