In a lab-scale upflow anaerobic sludge blanket reactor inoculated with granular sludge from a full-scale wastewater treatment plant treating paper mill wastewater, methanethiol (MT) was degraded at 30°C to H 2 S, CO 2 , and CH 4 . At a hydraulic retention time of 9 h, a maximum influent concentration of 6 mM MT was applied, corresponding to a volumetric loading rate of 16.5 mmol liter ؊1 day ؊1 . The archaeal community within the reactor was characterized by anaerobic culturing and denaturing gradient gel electrophoresis analysis, cloning, and sequencing of 16S rRNA genes and quantitative PCR. Initially, MT-fermenting methanogenic archaea related to members of the genus Methanolobus were enriched in the reactor. Later, they were outcompeted by Methanomethylovorans hollandica, which was detected in aggregates but not inside the granules that originated from the inoculum, the microbial composition of which remained fairly unchanged. Possibly other species within the Methanosarcinacaea also contributed to the fermentation of MT, but they were not enriched by serial dilution in liquid media. The archaeal community within the granules, which was dominated by Methanobacterium beijingense, did not change substantially during the reactor operation. Some of the species related to Methanomethylovorans hollandica were enriched by serial dilutions, but their growth rates were very low. Interestingly, the enrichments could be sustained only in the presence of MT and did not utilize any of the other typical substrates for methylotrophic methanogens, such as methanol, methyl amine, or dimethylsulfide.Organosulfur compounds present in petroleum and other fossil fuels are receiving considerable attention because of their potential direct and indirect negative effects on the environment. Thiols and H 2 S, the major (organo)sulfur compounds in liquefied petroleum gas (LPG), are extracted using alkaline solutions. Currently, thiol-containing LPG (mainly methanethiol [MT]) is treated by the Merox process, in which H 2 S and thiols are extracted from the LPG separately. The thiols are then catalytically oxidized to water-insoluble disulfide oil (33).Alternatively, H 2 S and thiols can be extracted simultaneously from LPG, which results in a solvent stream loaded with H 2 S, MT, and at least 1 M of Na ϩ . The thiols present in the solvent are converted in an anaerobic bioreactor to H 2 S, CO 2 , and CH 4 (26). In a second reactor, H 2 S is biologically oxidized to elemental sulfur, a process that has been studied in detail (7,8) and is already being applied. After separation of the elemental sulfur, the solvent is reused for the extraction process.Under anaerobic conditions, MT is fermented by methanogens, but oxidation by sulfate reducers has also been reported (15,30). Kiene et al. showed that sulfate reducers compete with methanogens for MT and dimethylsulfide (DMS), but only at concentrations below 10 M (10). Theoretically, conversion of MT to acetate or to H 2 , CO 2 , and H 2 S, as described for methanol (20,21), is also possibl...