Purple non-sulfur photosynthetic bacterium Rhodobacter sphaeroides KD131 wild type (wt) and its PHB synthase deleted-mutant P1 were evaluated for hydrogen (H(2)) production from acetate and butyrate, the most abundant liquid end products of dark fermentation. In the presence of glutamate (8 mM), 60 mM of acetate and 30 mM of butyrate were degraded down to 41.5% and 24.0%, respectively, and achieved a H(2) yield (HY) of 0.65 mol H(2)/mol acetate- and 2.50 mol H(2)/mol butyrate-consumed, while 30 mM succinate exhibited an HY of 3.29 mol H(2)/mol substrate-consumed. The order of HY observed was inversely related to poly-(3-hydroxybutyrate) (PHB) content and pH increase in the broth. When mutant P1 was used, in spite of depressed cell growth and lower substrate degradation compared to those observed in strain KD131 wt, higher H(2) production was observed, achieving around two-fold increase of MY in both acetate and butyrate. A pH control to 7.0 during fermentation was effective in increasing substrate degradation and decreasing PHB content, thereby significantly increasing H(2) production. When pH was controlled to 7.0, strain KD131 wt evolved more H(2) by 2.36 and 1.70 folds in the acetate- and succinate-medium, respectively, compared to those observed in without pH control. The highest H(2) production was observed when the mutant P1 was photo-fermented with a pH control to 7.0 in the medium containing acetate-(NH(4))(2)SO(4). It seemed that pH control had an effect not only on the depressed production of PHB but also on soluble microbial products and secondary metabolites, which would compete with H(2) production in expending reducing power. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.