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The degradation of organic matter to CH<sub>4</sub> and CO<sub>2</sub> was investigated in three different boreal peatland systems in Finland, a mesotrophic fen (MES), an oligotrophic fen (OLI), and an ombrotrophic peat (OMB). MES had similar production rates of CO<sub>2</sub> and CH<sub>4</sub>, but the two nutrient-poor peatlands (OLI and OMB) produced in general more CO<sub>2</sub> than CH<sub>4</sub>. <i>δ</i><sup>13</sup>C analysis of CH<sub>4</sub> and CO<sub>2</sub> in the presence and absence methyl fluoride (CH<sub>3</sub>F), an inhibitor of acetoclastic methanogenesis, showed that CH<sub>4</sub> was predominantly produced by hydrogenotrophic methanogenesis and that acetoclastic methanogenesis only played an important role in MES. These results, together with our observations concerning the collective inhibition of CH<sub>4</sub> and CO<sub>2</sub> production rates by CH<sub>3</sub>F, indicate that organic matter was degraded through different paths in the mesotrophic and the nutrient-poor peatlands. In the mesotrophic fen, the major process is canonical fermentation followed by acetoclastic and hydrogenotrophic methanogenesis, while in the nutrient-poor peat, organic matter was apparently degraded to a large extent by a different path which finally involved hydrogenotrophic methanogenesis. Our data suggest that degradation of organic substances in the oligotrophic environments was incomplete and involved the use of organic compounds as oxidants.