A mutant strain of the anaerobic purple sulfur bacterium Thiocapsa roseopersicina, containing only nitrogenase as a functionally active enzyme for H-2 generation was utilized to study the production of H-2 from organic acids (acetate, pyruvate and succinate). Two types of potential substrates for H-2 production, thiosulfate and salts of various organic acids, were compared under photoheterotrophic growth conditions. Thiosulfate proved to be the preferred electron donor for T. roseopersicina; the consumption of organic acids became pronounced only following depletion of the thiosulfate supply. The system is suitable for the generation of H-2 from effluents of heterotrophic dark fermentation processes or waste streams rich in inorganic reduced sulfur compounds and/or simple organic acids. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.