Boosted by the high temperatures in which vinasse is generated (90 degrees C-100 degrees C), this study evaluated the effect of an extreme thermophilic condition (70 degrees C) on sugarcane vinasse Dark Fermentation (DF) in an Anaerobic Structured Bed Reactor (ASTBR). Four hydraulic retention times (HRT) (19, 15, 12 and 8 h) were evaluated. Higher HRT resulted in a greater H-2 production rate (690 mLH(2).d(-1).L-1), higher yields (1.8 molH(2).mol(Glucose)(-1)) and greater stability. The extreme temperature inhibits microorganisms' extracellular polymer production, thus leading to a disperse growth, preventing excess biomass accumulation, which was previously reported as the main drawback in H-2 production at lower temperatures. The ASTBR higher void index is also responsible for lower biomass/solids retention. The H-2 production main route was through the lactic/acetic acid pathway, which is highly reliant on the pH of fermentation broth. The main genus involved in H-2 production at 70 degrees C were Clostridium, Pectinatus, Megasphaera and Lactobacillus. (C) 2019 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.