This study evaluates the effect of pH (4-7) on fermentative biohydrogen production by utilizing three isolated Clostridium species. Fermentative batch experiments show that the maximum hydrogen yield for Clostridium butyricum CGS2 (1.77 mmol/mmol glucose) is achieved at pH 6, whereas a high hydrogen production with Clostridium beijerinckii L9 (1.72 mmol/mmol glucose) and Clostridium tyrobutyricum FYa102 (1.83 mmol/mmol glucose) could be achieved under uncontrolled pH conditions (initial pH of 6.4-6.6 and final pH of 4-4.2). Low hydrogen yields (0-0.6 mmol/mmol glucose) observed at pH 4 are due likely to inhibitory effects on the microbial growth, although a low pH can be thermodynamically favorable for hydrogen production. The low hydrogen yields (0.12-0.64 mmol/mmol glucose) observed at pH 7 are attributed not only to thermodynamically unfavorable, but also metabolically unfavorable for hydrogen production. The relatively high levels of lactate, propionate, or formate observed at pH 7 reflect presumably the high enzymatic activities responsible for their production, together with the low hydrogenase activity, resulting in a low hydrogen production. A correlation analysis of the data from present and previous studies on biohydrogen production with pure Clostridium cultures and mixed microflora indicates a close relation between the hydrogen yield (Ye,) and the (Y(H2))/(2(Y(HAc) + Y(HBu))) ratio, with the observed correlation coefficient (0.787) higher than that (0.175) between Y(H2) and the molar ratio of butyrate to acetate (B/A). Based on the (Y(H2))/(2(Y(HAc) + Y(HBu))) ratios observed at different pHs, a control of pH at 5.5-6.8 would seem to be an effective means to enhance the fermentative biohydrogen production. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.