ABSTRACTDegradation of dimethyl sulfide and methanethiol in slurries prepared from sediments of minerotrophic peatland ditches were studied under various conditions. Maximal aerobic dimethyl sulfide-degrading capacities (4.95 nmol per ml of sediment slurry · h⁻¹), measured in bottles shaken under an air atmosphere, were 10-fold higher than the maximal anaerobic degrading capacities determined from bottles shaken under N₂or H₂atmosphere (0.37 and 0.32 nmol per ml of sediment slurry · h⁻¹, respectively). Incubations under experimental conditions which mimic the in situ conditions (i.e., not shaken and with an air headspace), however, revealed that aerobic degradation of dimethyl sulfide and methanethiol in freshwater sediments is low due to oxygen limitation. Inhibition studies with bromoethanesulfonic acid and sodium tungstate demonstrated that the degradation of dimethyl sulfide and methanethiol in these incubations originated mainly from methanogenic activity. Prolonged incubation under a H₂atmosphere resulted in lower dimethyl sulfide degradation rates. Kinetic analysis of the data resulted in apparentK_mvalues (6 to 8 μM) for aerobic dimethyl sulfide degradation which are comparable to those reported forThiobacillusspp.,Hyphomicrobiumspp., and other methylotrophs. ApparentK_mvalues determined for anaerobic degradation of dimethyl sulfide (3 to 8 μM) were of the same order of magnitude. The low apparentK_mvalues obtained explain the low dimethyl sulfide and methanethiol concentrations in freshwater sediments that we reported previously. Our observations point to methanogenesis as the major mechanism of dimethyl sulfide and methanethiol consumption in freshwater sediments.