Elevated temperatures (52, 60 and 65 degrees C) were used to enrich hydrogen producers on cellulose from cow rumen fluid. Methanogens were inhibited with two different heat treatments. Hydrogen production was considerable at 60 degrees C with the highest H(2) yield of 0.44 mol-H(2) mol-hexose(-1) (1.93 mol-H(2) mol-hexose-degraded(-1)) as obtained without heat treatment and with acetate and ethanol as the main fermentation products. H(2) production rates and yields were controlled by cellulose degradation that was at the highest 21%. The optimum temperature and pH for H(2) production of the rumen fluid enrichment culture were 62 degrees C and 7.3, respectively. The enrichments at 52 and 60 degrees C contained mainly bacteria from Clostridia family. At 52 degrees C, the bacterial diversity was larger and was not affected by heat treatments. Bacterial diversity at 60 degrees C remained similar between heat treatments, but decreased during enrichment. At 60 degrees C, the dominant microorganism was Clostridium stercorarium subsp. leptospartum. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.