In this contribution, the effect of the presence of a presumed inert gas like ${\mathrm{N}}_2$ in the feed gas on the biological methanation of hydrogen and carbon dioxide with Methanothermobacter marburgensis was investigated. ${\mathrm{N}}_2$ can be found as a component besides ${\mathrm{CO}}_2$ in possible feed gases like mine gas, weak gas, or steel mill gas. To determine whether there is an effect on the biological methanation of ${\mathrm{CO}}_2$ and ${\mathrm{H}}_2$ from renewable sources or not, the process was investigated using feed gases containing ${\mathrm{CO}}_2$ , ${\mathrm{H}}_2$ , and ${\mathrm{N}}_2$ in different ratios, depending on the ${\mathrm{CO}}_2$ content. A possible effect can be a lowered conversion rate of ${\mathrm{CO}}_2$ and ${\mathrm{H}}_2$ to ${\mathrm{CH}}_4$ . Feed gases containing up to 47% ${\mathrm{N}}_2$ were investigated. The conversion of hydrogen and carbon dioxide was possible with a conversion rate of up to 91% but was limited by the amount of ${\mathrm{H}}_2$ when feeding a stoichiometric ratio of 4:1 and not by adding ${\mathrm{N}}_2$ to the feed gas.