Dark fermentation of lignocellulosic residues is a process of interest producing simultaneously biohydrogen and valuable industrial end-products (volatile fatty acids, organic acids or alcohols). Theoretically, wet (<15% Total Solid (TS)) and dry (>20% TS) fermentation technologies can be operated at mesophilic (35°C) and thermophilic (55°C) temperatures [1]. Even though high temperatures favour substrate hydrolysis, high TS content processes could lead to microbial limitations [2]. The aim of this study was to assess the impact of water content reduction on dark fermentation processes at mesophilic and thermophilic temperatures. An experiment was set-up to study the impact of TS content from wet to dry conditions (10% to 34% TS) in discontinuous anaerobic batch tests operated with wheat straw as substrate and under mesophilic and thermophilic conditions. For both temperatures, more than 15 NmH2.gTS-1 were obtained in wet fermentation, and the results indicated a decrease of substrate conversion in dry fermentation. Under mesophilic conditions, similar metabolic pathways of hydrogen production occurred from 10% to 28% TS, after what a metabolic shift occurred towards butyric acid production. Under thermophilic conditions, a lactic acid metabolism appeared at 19% TS, which was attributed to the emergence of several microorganisms (Clostridium and Bacillus species). In this study, it was concluded that dry mesophilic fermentation presented a metabolic shift only when free water was lacking (28% TS), whereas in thermophilic environment, microbial competition turned in favour of lactic acid producers for the overall range of dry conditions (>19% TS). Further studies are in progress to better understand the different hydrogen inhibition levels at both temperatures.