Abstract The biological conversion of H2 and CO2 into CH4, using methanogenic archaea is an interesting technology for CO2 conversion, energy storage and biogas upgrading. For an industrial application of this process however, the optimization of the volumetric productivity and the product quality is an important issue. Since the reactants in this fermentation process are, unlike in most microbial fermentations, solely gasses, the gas liquid mass transfer is supposed to play an important role on the way to a higher volumetric productivity. This work aimed at investigating the effects of the gassing rate, the reactor pressure, as well as reactor design issues on the performance of Methanothermobacter marburgensis by using continuous cultures. Our results show that biological methanogenesis with M. marburgensis is gas limited. Maximum physiological capacity is not reached yet. The gassing rate influenced mainly the volumetric methane production rate (MER), the reactor pressure influenced mainly the offgas quality. Based on this information, we demonstrated how a combination of increased gas flow rate and increased reactor pressure can be used to reach high volumetric productivity at high offgas quality. Maximum MER was 950 mmol L−1 h−1 at a CH4 concentration of 60 Vol.-%, maximum CH4 concentration reached was 85 Vol.-% at a MER of 255 mmol L−1 h−1. The reactor design currently limits further increase in gas flow rate and reactor pressure. Therefore Interdisciplinary bridges from bioprocessing to chemical reactor design must be followed in the future to boot this promising bioprocess to gain biomethane via CO2 fixation.