Methanogenesis from the non-physiological C1 donors thioproline, thiazolidine, hexamethylenetetramine, formaldehyde (HCHO), and HOCH2-S-coenzyme M (CoM) was catalyzed by cell extracts of Methanobacterium thermoautotrophicum under a hydrogen atmosphere. Tetrahydromethanopterin (H4MPT) and HS-CoM were required in the reaction mixture. The non-physiological compounds were found to be in chemical equilibrium with HCHO, which has been shown to react spontaneously with H4MPT to form methylene-H4MPT, an intermediate of the methanogenic pathway at the formaldehyde level of oxidation. Highfield (360 MHZ) 1H and 13C nuclear magnetic resonance studies performed on the interaction between HCHO and HS-CoM showed that these compounds are in equilibrium with HOCH2-S-CoM and that the equilibrium is pH dependent. When methanogenesis from the non-physiological donors was followed under a nitrogen atmosphere, the C1 moiety from each compound underwent a disproportionation, forming methenyl-H4MPT+ and methane. The compounds tested served as substrates for the enzymatic synthesis of methenyl-H4MPT+.