The influence of different inoculum pretreatments (pH and temperature shocks) on mesophilic (37 degrees C) and thermophilic (55 degrees C) dark fermentative H-2 production from xylose (50 mM) and, for the first time, on the composition of the active microbial community was evaluated. At 37 degrees C, an acidic shock (pH 3, 24 h) resulted in the highest yield of 0.8 mol H2 mol(-1) xylose. The H-2 and butyrate yield correlated with the relative abundance of Clostridiaceae in the mesophilic active microbial community, whereas Lactobacillaceae were the most abundant non-hydrogenic competitors according to RNA-based analysis. At 55 degrees C, Clostridium and Thermoanaerobacterium were linked to H-2 production, but only an alkaline shock (pH 10, 24 h) repressed lactate production, resulting in the highest yield of 1.2 mol H-2 mol(-1) xylose. This study showed that pretreatments differentially affect the structure and productivity of the active mesophilic and thermophilic microbial community developed from an inoculum. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.