To understand the processes responsible for the presence of low-molecular-weight hydrocarbons (C1-C4) in anoxic environments, we studied sediments collected from an anaerobic estuarine mudflat. In these sediments methane (C1) was several orders of magnitude more abundant than all other C2-C4 hydrocarbons; the C1 (C2 + C3) ratio was ??? 13,000. Mean ethane/ethene and propane/propene ratios were 0.4 and 0.7, respectively. Production of C1-C4 hydrocarbons was monitored during prolonged incubation (7 months) of sediments at 27?? and 4??C. Samples stored at 27??C generated significant quantities of C1-C4 hydrocarbon gases. Incubation at 4??C inhibited production of these gases. Several bactericides were tested with respect to their ability to inhibit formation of gaseous hydrocarbons. Sodium azide, chloroform, and 2-bromoethanesulfonic acid effectively inhibited methane formation, but not ethene formation in dilute continuously-shaken sediment slurries. Zephiran chloride only caused partial inhibition of methanogenesis (46%) and ethene generation (34%) in these slurries. In experiments with more concentrated unshaken sediment slurries, however, zephiran chloride and sodium azide did not block formation of methane, ethane, or propane. Only storage at -10??C prevented production of these gases. These results indicate that C1-C4 hydrocarbons can be formed by low-temperature reactions, possibly mediated by microorganisms. ?? 1982.