This study evaluates the use of non-pretreated thermo-mechanical pulping (TMP) wastewater as a potential substrate for hydrogen production by dark fermentation. Batch incubations were conducted in a temperature gradient incubator at temperatures ranging from 37 to 80 degrees C, using an inoculum from a thermophilic, xylose-fed, hydrogen-producing fluidised bed reactor. The aim was to assess the short-term response of the microbial communities to the different temperatures with respect to both hydrogen yield and composition of the active microbial community. High throughput sequencing (MiSeq) of the reversely transcribed 16S rRNA showed that Thermoanaerobacterium sp. dominated the active microbial community at 70 degrees C, resulting in the highest hydrogen yield of 3.6 (+/- 0.1) mmol 112 g(-1) CODtot supplied. Lower hydrogen yields were obtained at the temperature range from 37 to 65 degrees C, likely due to consumption of the produced hydrogen by homoacetogenesis. No hydrogen production was detected at temperatures above 70 degrees C. Thermomechanical pulping wastewaters are released at high temperatures (50-80 degrees C), and thus dark fermentation at 70 degrees C could be sustained using the heat produced by the pulp and paper plant itself without any requirement for external heating. (C) 2018 Elsevier B.V. All rights reserved.