We determined the geochemical characteristics of sediments and measured rates of the anaerobic oxidation of methane (AOM) and sulfate reduction (SR) in samples collected near thermogenic (structure II) gas hydrate mounds and in areas lacking hydrates along the continental slope in the Gulf of Mexico. We used radiotracer (C-14 and S-35) techniques to determine rates of AOM and SR over depth in sediment cores. Abundant mats of white and orange Beggiatoa spp. were common in areas of active seepage and these sediments were enriched in hydrogen sulfide and methane. In cores collected from areas without Beggiatoa or hydrate, concentrations of redox metabolites showed little variation over depth and these sites were inferred to be areas of low seepage. Integrated AOM rates were low in Beggiatoa-free cores (<0.05 mmol m-2 day-1) and averaged 2.8 ± 4.6 mmol m-2 day-1 in seep cores that contained Beggiatoa or gas hydrate. Integrated SR rates were also low in Beggiatoa-free cores (<1 mmol m-2 day-1) and averaged 54 ± 94 mmol m-2 day-1 in cores with Beggiatoa or hydrate. Rates of SR generally exceeded rates of AOM and the two processes were loosely coupled, suggesting that the majority of SR at Gulf of Mexico hydrocarbon seep sites is likely fueled by the oxidation of other organic matter, possibly other hydrocarbons and oil, rather than by AOM. © 2004 Elsevier B.V. All rights reserved.