Dark fermentative hydrogen production from glucose by a thermophilic culture (33HL), enriched from an Icelandic hot spring sediment sample, was studied in two continuous-flow, completely stirred tank reactors (CSTR1, CSTR2) and in one semi-continuous, anaerobic sequencing batch reactor (ASBR) at 588C. The 33HL produced H 2 yield (HY) of up to 3.2 mol-H 2 /mol-glucose along with acetate in batch assay. In the CSTR1 with 33HL inoculum, H 2 production was unstable. In the ASBR, maintained with 33HL, the H 2 production enhanced after the addition of 6 mg/L of FeSO 4 Á 7H 2 O resulting in HY up to 2.51 mol-H 2 /molglucose (H 2 production rate (HPR) of 7.85 mmol/h/L). The H 2 production increase was associated with an increase in butyrate production. In the CSTR2, with ASBR inoculum and FeSO 4 supplementation, stable, high-rate H 2 production was obtained with HPR up to 45.8 mmol/h/L (1.1 L/h/ L) and HY of 1.54 mol-H 2 /mol-glucose. The 33HL batch enrichment was dominated by bacterial strains closely affiliated with Thermobrachium celere (99.8-100%). T. celere affiliated strains, however, did not thrive in the three open system bioreactors. Instead, Thermoanaerobacterium aotearoense (98.5-99.6%) affiliated strains, producing H 2 along with butyrate and acetate, dominated the reactor cultures. This culture had higher H 2 production efficiency (HY and specific HPR) than reported for mesophilic mixed cultures. Further, the thermophilic culture readily formed granules in CSTR and ASBR systems. In summary, the thermophilic culture as characterized by high H 2 production efficiency and ready granulation is considered very promising for H 2 fermentation from carbohydrates.