Sediments from mercury-contaminated and uncontaminated reaches of the Carson River, Nevada, were assayed for sulfate reduction, methanogenesis, denitrification, and monomethylmercury (MeHg) degradation. Demethylation of [¹⁴C]MeHg was detected at all sites as indicated by the formation of ¹⁴CO₂ and ¹⁴CH₄. Oxidative demethylation was indicated by the formation of ¹⁴CO₂ and was present at significant levels in all samples. Oxidized/reduced demethylation product ratios (i.e., ¹⁴CO₂/¹⁴CH₄ ratios) generally ranged from 4.0 in surface layers to as low as 0.5 at depth. Production of ¹⁴CO₂ was most pronounced at sediment surfaces which were zones of active denitrification and sulfate reduction but was also significant within zones of methanogenesis. In a core taken from an uncontaminated site having a high proportion of oxidized, coarse-grain sediments, sulfate reduction and methanogenic activity levels were very low and ¹⁴CO₂ accounted for 98% of the product formed from [¹⁴C]MeHg. There was no apparent relationship between the degree of mercury contamination of the sediments and the occurrence of oxidative demethylation. However, sediments from Fort Churchill, the most contaminated site, were most active in terms of demethylation potentials. Inhibition of sulfate reduction with molybdate resulted in significantly depressed oxidized/reduced demethylation product ratios, but overall demethylation rates of inhibited and uninhibited samples were comparable. Addition of sulfate to sediment slurries stimulated production of ¹⁴CO₂ from [¹⁴C]MeHg, while 2-bromoethanesulfonic acid blocked production of ¹⁴CH₄. These results reveal the importance of sulfate-reducing and methanogenic bacteria in oxidative demethylation of MeHg in anoxic environments.