Sediment oxygen demands and water-atmosphere methane emissions of a highly productive, freshwater wetland on the floodplain of the River Murray in south-eastern Australia were quantified over a 14 month period in 1993–1994. Total sediment oxygen demands ranged from 1.3 to 3.3 mmol m−2 h−1, of which < 3 to 90% was due to chemical oxygen demand. Methane emissions ranged from < 0.01 mmol m−2 h−1 in winter to 2.75 mmol m−2 h−1 in summer. Methanogenesis accounted for at least 60% of the combined aerobic and methanogenic carbon flux in sediments from Eleocharis sphacelata beds, and at least 30% and 40% of the combined flux in sediments from Myriophyllum sp. beds and Vallisneria gigantea beds, respectively. In vitro incubations, using additions of sulfate and of molybdate, failed to indicate unequivocally competition for substrates between sulfate-reducing and methanogenic bacteria. However, in vitro methanogenesis was strongly inhibited by nitrate, suggesting an interaction between benthic methanogens and denitrifying or other nitrate-reducing bacteria. Fe3+ decreased in vitro methanogenesis by 16–49% during January, February and March 1994; oxidation of organic matter at the expense of the reduction of ferric ions could be a significant route for detritus processing in Eleocharis-bed sediments in the warmer months. Methanogenesis was increased consistently by additions of some low molecular weight substrates, such as acetate, but not by others, such as methanol, propionate and trimethylamine. Complex polymeric substrates, such as cellulose, starch and aquatic plant matter, increased in vitro methanogenesis rapidly and markedly. Despite this, no relationship between methane emissions and benthic cellulase activity was observed in the field. Methanogenesis was strongly temperature dependent, being maximal at 30 to 40°C and minimal at 5°C, thereby explaining the strong seasonality observed in methane emissions in situ.