In this study, the H(2) production and chemical oxygen demand (COD) removal performances of a thermophilic upflow anaerobic sludge blanket (UASB) reactor with online monitoring system were investigated. The online monitoring system enabled a rapid monitoring and timely control of the process. As a consequence, high operating stability was achieved despite of the varied hydraulic retention time (HRT) during 310-day operation. The COD efficiency remained at above 98%, and the hydrogen yield fluctuated slightly within the range of 2.42-3.06 mol H(2) mol(-1) sucrose. Thermophilic H(2)-producing granules were successfully cultivated in this reactor, which showed better physical and microbial properties than floc sludge and higher H(2) production rate than mesophilic granules. An analysis of the microbial growth kinetics further demonstrated a possibly higher synthesis and metabolism activity of microbes in the thermophilic granule state. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.