Background: Biomethanation is a promising solution to upgrade the CH 4 content in biogas. This process consists in the injection of H 2 into an anaerobic digester, using the capacity of indigenous hydrogenotrophic methanogens for converting the injected H 2 and the CO 2 generated from the anaerobic digestion process into CH 4. However, the injection of H 2 could cause process disturbances by impacting the microbial communities of the anaerobic digester. Better understanding on how the indigenous microbial community can adapt to high H 2 partial pressures is therefore required. Results: Seven microbial inocula issued from industrial bioprocesses treating different types of waste were exposed to a high H 2 partial pressure in semi-continuous reactors. After 12 days of operation, even though both CH 4 and volatile fatty acids (VFA) were produced as end products, one of them was the main product. Acetate was the most abundant VFA, representing up to 94% of the total VFA production. VFA accumulation strongly anti-correlated with CH 4 production according to the source of inoculum. Three clusters of inocula were distinguished: (1) inocula leading to CH 4 production, (2) inocula leading to the production of methane and VFA in a low proportion, and (3) inocula leading to the accumulation of mostly VFA, mainly acetate. Interestingly, VFA accumulation was highly correlated to a low proportion of archaea in the inocula, a higher amount of homoacetogens than hydrogenotrophic methanogens and, the absence or the very low abundance in members from the Methanosarcinales order. The best methanogenic performances were obtained when hydrogenotrophic methanogens and Methanosarcina sp. co-dominated all along the operation. Conclusions: New insights on the microbial community response to high H 2 partial pressure are provided in this work. H 2 injection in semi-continuous reactors showed a significant impact on microbial communities and their associated metabolic patterns. Hydrogenotrophic methanogens, Methanobacterium sp. or Methanoculleus sp. were highly selected in the reactors, but the presence of co-dominant Methanosarcinales related species were required to produce higher amounts of CH 4 than VFA.