The effect of temperature and hydraulic retention time (HRT) on the homoacetogenesis and on .the morphological characteristics of hydrogen producing granules was investigated. Hydrogen was produced using an expanded granular sludge blanket (EGSB) reactor, fed with glucose and Larabinose, under mesophilic (37ºC), thermophilic (55ºC), and hyperthermophilic (70ºC) conditions. Apparent homoacetogenesis was observed only at mesophilic conditions. Glucose was utilized preferentially over L-arabinose at thermophilic and hyper-thermophilic conditions. Higher yields of hydrogen production were ...