The present study investigated the inhibitory effects of saturated fatty acids on methanogenesis in Archaea, and whether or not competitive inactivation of the methanogens’ coenzyme M (HS-CoM) is involved in the inhibition. Strains tested in batch cultures were Methanosarcina barkeri, Methanosarcina mazei, Methanococcus voltae, all incubated at 37°C, and Methanothermobacter thermoautotrophicus, incubated at 65°C. The fatty acids C10, C12, C14 and C18 were supplemented at 1 mg . ml–1 in cultivation medium. The methanogens were susceptible to C10 and C12, and less so to C14. Only M. thermoautotrophicus was affected by C18. In M. mazei cultures, excessive HS-CoM did not prevent the action of C14 which might suggest that competitive inhibition of HS-CoM is not the reason for the SFA-induced effect on methanogenesis. The results indicate that, as a prerequisite to inhibit methanogenesis in Archaea, medium- and long-chain saturated fatty acids have to be at least partially molten.