The syntrophic oxidation of primary fermentation products like ethanol, butyrate, or propionate to methanogenic substrates (H2/CO2, formate, acetate) is a thermodynamic bottle-neck during the mineralization of organic matter in anoxic habitats like peatlands. Despite the importance of syntrophic oxidations to the emission of methane, little is known about the microbial community that catalyzes these syntrophic processes in peat. The consumption of butyrate, ethanol, or propionate by anoxic peat slurries at 5°C and 15°C yielded methane and CO2 as the sole accumulating products, indicating that the intermediates H2, formate, and acetate were scavenged effectively by syntrophic methanogenic consortia. 16S rRNA stable isotope probing identified novel species/strains of Pelobacter and Syntrophomonas that syntrophically oxidized ethanol and butyrate, respectively. Propionate was syntrophically oxidized by novel species of Syntrophobacter and Smithella, genera that use different propionate-oxidizing pathways. Taxa not known for a syntrophic metabolism may have been involved in the oxidation of butyrate (Telmatospirillum-related) and propionate (unclassified Bacteroidetes and unclassified Fibrobacteres). Gibbs free energies (ΔGs) for syntrophic oxidations of ethanol and butyrate were more favorable than ΔGs for syntrophic oxidation of propionate. As a result of the thermodynamic constraints, acetate transiently accumulated in ethanol and butyrate treatments but not in propionate treatments. Aceticlastic methanogens (Methanosarcina, Methanosaeta) appeared to outnumber hydrogenotrophic methanogens (Methanocella, Methanoregula), reinforcing the likely importance of aceticlastic methanogenesis to the overall production of methane. ΔGs for acetogenesis from H2-CO2 approximated -20 kJ∙mol-1 when acetate concentrations were low, indicating that acetogens may have contributed to the flow of carbon and reductant towards methane.