Methane is produced in the rumen by methanogenic archaea - a singular group of microbes. Although the basic biochemistry of methanogenesis has been elucidated, little is known about other aspects of their physiology. The objective of this work was to improve our knowledge of methanogens’ metabolism at the intra-cellular level. We pursued an approach that combines in-vitro kinetic studies with molecular biology and biochemical measures. Three methanogenic species, Methanobrevibacter smithii (Ms), Methanobacterium formicium (Mf), and Methanobrevibacter wolinii (Mw) were grown in Balch medium with H2/CO2 overpressure (~3000 mbar). Optical density (OD), gas pressure and composition were measured daily over a 2-week period. The quantification of heat flow from actively growing cultures was done by isothermal microcalorimetry. All three species fully used the H2 within 3 days and no significant difference was observed in the amount of CH4 (on average 345 µmoles) produced at the end of incubation. However, ODmax varied between 0.229, 0.573 and 0.585 for Mf, Mw and Ms, respectively. The raw thermal power signal for all methanogens was composed of two main peaks: one rather intense and short-lived followed by shallow and longer peak. However, the curves layout was species-dependant: the first peak reached maximum values of 15µW, 18µW and 20µW at 3, 4 and 5 days after inoculation, respectively, for Mw, Ms and Mf; whereas the shape and the intensity of the second peak greatly differed between species. Total heat was similar for Ms and Mw (6.7 and 6.6 J) but lower for Mf (5.6 J), which was consistent with OD values. Species-specific metabolism kinetics revealed by this study suggest differentiation within methanogenesis niche- specialists that may be important to understand and mitigate emissions. Methanogens physiology will be further studied using transcriptomic and metabolomics approaches to identify the underlying metabolic processes.