In this study we investigated the revivability of a continuous biological hydrogen producing reactor after a period of feed interruption. Before the feed interruption, the hydrogen production yield was 1.36 mol H(2)/mol glucose with butyric acid and acetic acid as the main metabolic products. However, after feed interruption, butyric acid formation completely stopped and the hydrogen yield decreased to 0.29 mol H(2)/mol glucose. Lactic acid, ethanol and acetic acid became the main metabolites after re-start up. Reduction of organic loading rate together with increasing the pH after the feed interruption resulted in an increase in the hydrogen yield to 0.7 mol H(2)/mol glucose. The microbial community dynamics showed complete elimination of Clostridium affiliated strains and predominance of Lactobacillus affiliated strains after the re-start up of the reactor. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.