The contribution of Phragmites australis (Cav.) Trin. ex Steud. (common reed) litter as an origin of CH ₄-C was studied in a microcosm experiment. ¹⁴C-labelled, dried and ground P. australis root and shoot litter was buried in waterlogged, planted or non-planted fen microcosms. The evolution of ¹⁴CO ₂ and ¹⁴CH ₄ from the pots was monitored during the 35-day experiment. The ¹⁴C activity in the shoots and roots of the plants, soil, and inundation water was also analysed at the end of the experiment. Up to 40% of the released CH ₄-C originated from the added litter, whereas the rest originated from old soil organic matter. The comparison of planted and non-planted pots suggested that the contribution of recent plant-derived C (i.e. root exudates) to CH ₄ emission was negligible. The proportion of litter-derived CH ₄-C was significantly higher in the planted pots, suggesting that the presence of plants enhanced the formation of CH ₄ from litter. The major part of the initial ¹⁴C activity was recovered from the soil. About 3% was recovered from the inundation water, about 10% was emitted as CO ₂, and only <0.01% as CH ₄. However, these results demonstrated that plant litter and old soil organic matter are the major sources of CH ₄-C in fens during the early growth stage of P. australis.