In this study, the performance of anaerobic hydrogen-producing monocultures of the extreme thermophiles Caldicellulosiruptor owensensis and C. kristjanssonii as well as cultures of thermophilic microflora enriched form local compost and wastewater sludge samples was investigated. Hydrogen production efficiencies were compared to that of the extreme thermophile C. saccharolyticus, which was used here as a benchmark organism. Some of the tested thermophiles were subsequently grown together in a continuous stirred tank reactor (CSTR) where the microbial population was analyzed at different HRTs through denaturing gradient gel electrophoresis (DGGE) of 16s rDNA. Our aim is to obtain a stable, efficient hydrogen-producing consortium that has not been naturally met before. This work is part of the Integrated Project, HYvolution, for non-thermal production of hydrogen from biomass, funded in the EU FP6.