Three different Rhodobacter sphaeroides (RS) strains (RS-NRRL, RS-DSMZ and RS-RV) and their combinations were used for light fermentation of dark fermentation effluent of ground wheat containing volatile fatty acids (VFA). in terms of cumulative hydrogen formation, RS-NRRL performed better than the other two strains producing 48 ml H(2) in 180 h. However, RS-RV resulted in the highest hydrogen yield of 250 ml H(2) g(-1) TVFA. Specific hydrogen production rate (SHPR) with the RS-NRRL was also better in comparison to the others (13.8 End H(2) g(-1) biomass h(-1)). When combinations of those three strains were used, RS-RV + RS-DSMZ resulted in the highest cumulative hydrogen formation (90 ml H(2) in 330 h). However, hydrogen yield (693 ml H(2) g(-1) TVFA) and SHPR (12.1 ml H(2) g(-1) biomass h(-1)) were higher with the combination of the three different strains. on the basis of Gompertz equation coefficients mixed culture of the three different strains gave the highest cumulative hydrogen and formation rate probably due to synergistic interaction among the strains. The effects of initial TVFA and NH(4)-N concentrations on hydrogen formation were investigated for the mixed culture of the three strains. The optimum TVFA and NH(4)-N concentrations maximizing the hydrogen formation were determined as 2350 and 47 mg L(-1), respectively. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.