Pure cultures of H-2/CO2- and formate-utilizing methanogens or mixed consortia of sewage sludge generated some formate from H-2/CO2 at H-2 partial pressure in the gas phase above 200 kPa. At decreasing H-2 partial pressure the formate was taken up again and converted to methane. If methanogenesis was inhibited by bromoethanesulphonic acid (BESA) or a high redox potential (-180 to -200 mV), formate-utilizing methanogens produced high amounts of formate from H-2/CO2. No formate was excreted by the species, which could only utilize H-2/CO2 for methanogenesis. In contrast, H-2 formation from formate was observed in cultures of Methanobacterium thermoformicicum and M. formicicum. Measurable amounts were, however, only formed if its immediate utilization for methane production was inhibited by BESA. In the light of the data on formate formation from H-2/CO2 and its re-utilization by all formate-utilizing methanogens, the concept of interspecies formate transfer of Thiele and Zeikus should be reconsidered. In pure cultures of methanogens or complex ecosystems with excess H-2, formate formation seemed to serve more as a means of disposal of surplus reducing power than for H-2 transfer.