Hydrogen produced from periodic excess of electrical energy may be added to biogas reactors where it is converted to CH 4 that can be utilized in the existing energy grid. The major challenge with this technology is gas-to-liquid mass transfer limitation. The microbial conversions in reactors designed for hydrogenotrophic methanogenesis were studied with microsensors for H 2 , pH, and CO 2 . The H 2 consumption potential was dependent on the CO 2 concentration, but could partially recover after CO 2 depletion. Reactors with 3-dimensional biofilm carrier material and a large gas headspace allowed for a methanogenic biofilm in direct contact with the gas phase. A high density of Methanoculleus sp. in the biofilm mediated a high rate of CH 4 production, and it was calculated that a reactor filled with 75% carrier material could mediate a biogas upgrading from 50 to 95% CH 4 within 24 h when an equivalent amount of H 2 was added.