In the present study the methanogenic potential of formate (HCOO(-)) during thermophilic anaerobic digestion was investigated. After appropriate conditions for methanogenesis (HCOO(-) and inoculum concentration, pH and duration of incubation) were assessed, an experiment with initial 31 replicates was run. Diluted fermenter sludge was used as inoculum, and process parameters including the pH, quality and quantity of the produced biogas and the concentrations of volatile fatty acids and HCO(3) (-) were determined. Remarkably, after 5 days of incubation the highest CH(4) production was calculated for a HCOO(-) concentration of 200 mmol L(-1), a concentration, however, which might not occur in situ. During the phase of high CH(4) production HCOO(-) was degraded with a rate of 1.5 mmol L(-1) h(-1), and distinct changes of Gibbs free energy for several reactions were observed. Based on denaturing high-performance liquid chromatography, denaturing gradient gel electrophoresis, and additional subsequent sequencing approaches the hydrogenotrophic Methanothermobacter wolfeii was the dominant methanogen responsible for CH(4) production. Further confirmation was achieved due to the detection of autofluorescing rods with a size of up to ~3 µm, which were often arranged in pairs and chains. It was shown that even high concentrations of HCOO(-) are readily degraded, which might lead to an underestimation of both, the concentration and thus, the importance of HCOO(-) in anaerobic digestion.