A series of tests on the fermentation of vegetable kitchen wastes to produce hydrogen were conducted at pHs of 5.5, 6.0, 6.5, and 7.0 under a thermophilic condition. Microorganisms used in fermentation were enriched from kitchen-waste composts, and a high vegetable-content kitchen-waste was utilized. The initial substrate to microorganism ratio was kept at 10 g COD/g VSS under a thermophilic (55 degrees C) condition. A maximum specific hydrogen production rate of 0.48 mmol H-2/g VSS/h was achieved at pH = 6.0, and the highest hydrogen yield of 0.57 mmol H-2/g COD occurred at pH = 7.0. No hydrogen production occurred at pH = 5.5. At pHs controlled at 6.0, 6.5, and 7.0, the major volatile acid produced was butyrate. Using a terminal restriction fragment length polymorphism (TRFLP) analysis for microbial identification, T-RFs of 227 bps were found to be a dominant population when hydrogen was produced efficiently at pHs of 6.0, 6.5, and 7.0. Crown Copyright (C) 2008 Published by Elsevier Ltd on behalf of International Association for Hydrogen Energy. All rights reserved.