Power-to-gas allows conversion of surplus electricity to methane when CO2 is available, which becomes an important technology for carbon capture, utilization and sequestration, as well as for increasing the flexibility of electricity production from renewable energy resources such as wind and solar energy. H2/CO2 biomethanation is a potentially promising alternative to the conversion of H2/CO2 to methane without limitation of variable hydrogen production. To identify mixed culture-based metabolic pathways of H2/CO2 under the mesophilic (35 °C) and thermophilic (55 °C) conditions, two specific inhibitors, 2-bromoethane sulfonate (BES) and vancomycin were employed in this experimental study. The combination of hydrogenotrophic and homoacetogenesis-acetoclastic methanogenesis makes up the pathway for the mesophilic cultivated microbial consortia. 16S rRNA gene analysis indicates that abundant Bacteria, Methanobacterium and Methanosaeta play important role in the conversion. Further analysis shows close collaboration between microorganisms by the formation of microbial clustering and the production of humic acids. The detailed metabolic mechanisms further confirm a diverse biomethanation network under the mesophilic condition. While under the thermophilic condition, the H2/CO2 biomethanation is fully dominated by the direct hydrogenotrophic methanogenesis mainly with Methanothermobacter, which is straightforward but more efficient.