Background: Commercial biogas upgrading facilities are expensive and consume energy. Biological biogas upgrading may serve as a low-cost approach because it can be easily integrated with existing facilities at biogas plants. The microbial communities found in anaerobic digesters typically contain hydrogenotrophic methanogens, which can use hydrogen (H 2) as a reducing agent for conversion of carbon dioxide (CO 2) into methane (CH 4). Thus, biological biogas upgrading through the exogenous addition of H 2 into biogas digesters for the conversion of CO 2 into CH 4 can increase CH 4 yield and lower CO 2 emission. Results: The addition of 4 mol of H 2 per mol of CO 2 was optimal for batch biogas reactors and increased the CH 4 content of the biogas from 67 to 94%. The CO 2 content of the biogas was reduced from 33 to 3% and the average residual H 2 content was 3%. At molar H 2 :CO 2 ratios > 4:1, all CO 2 was converted into CH 4 , but the pH increased above 8 due to depletion of CO 2 , which negatively influenced the process stability. Additionally, high residual H 2 content in these reactors was unfavourable, causing volatile fatty acid accumulation and reduced CH 4 yields. The reactor microbial communities shifted in composition over time, which corresponded to changes in the reactor variables. Numerous taxa responded to the H 2 inputs, and in particular the hydrogenotrophic methanogen Methanobacterium increased in abundance with addition of H 2. In addition, the apparent rapid response of hydrogenotrophic methano-gens to intermittent H 2 feeding indicates the suitability of biological methanation for variable H 2 inputs, aligning well with fluctuations in renewable electricity production that may be used to produce H 2. Conclusions: Our research demonstrates that the H 2 :CO 2 ratio has a significant effect on reactor performance during in situ biological methanation. Consequently, the H 2 :CO 2 molar ratio should be kept at 4:1 to avoid process instability. A shift toward hydrogenotrophic methanogenesis was indicated by an increase in the abundance of the obligate hydrogenotrophic methanogen Methanobacterium.