The characterization of hydrogenotrophic methanogens by using analytical bioprocess technology holds great potential for the development of biological gas conversion processes. Therefore, we aimed to investigate the influence of liquid dilution rate and total gassing rate on the performance of Methanothermobacter marburgensis grown in continuous culture on H-2/CO2. By increasing medium dilution rate M. marburgensis biomass concentration decreased while specific methane and specific water productivity increased. Low liquid dilution rates resulted in highest volumetric uptake and production rates. Steady state cultivations were limited in the mass transfer from the gas to the liquid phase. By quantification of biological water production we were able to give additional insight into maintenance requirements of M. marburgensis. At higher growth rates, water production can be used as an additional on-line signal to quantify biological activity, as it is directly correlated to methane productivity. We demonstrate the usefulness of dynamic investigations, such as the medium dilution shift experiment, in order to physiologically characterize hydrogenotrophic methanogens for its maximum specific methane productivity. Process development can focus on the optimization of the gas-liquid mass transfer, until this physiological limit is reached. (C) 2011 Elsevier Ltd. All tights reserved.