Degradational studies of methanopterin, a coenzyme involved in methanogenesis, are reported. The results of these studies are in full accordance with the proposed structure of methanopterin as N‐[1′‐(2″‐amino‐4″‐hydroxy‐7″‐methyl‐6″‐pteridinyl)ethyl]‐4‐[2′,3′,4′,5′‐tetrahydroxypent‐1′‐yl(5′‐1″)O‐α‐ribofuranosyl‐5″‐phosphoric acid] aniline in which the phosphate group is esterified with α‐hydroxyglutaric acid. Acid hydrolysis of methanopterin cleaved the 5′→ 1″ glycosidic bond and yielded a ‘hydrolytic product’ which was identified as N‐[1′‐(2″‐amino‐4″‐hydroxy‐7″‐methyl‐6″‐pteridinyl)ethyl]‐4‐[2′, 3′,4′, 5′‐tetrahydroxypent‐1′‐yl]aniline. Alkaline permanganate oxidation of methanopterin yielded 7‐methylpterin‐6‐carboxylic acid. Catalytic (or enzymatic) hydrogenation of methanopterin gave a mixture of 6‐ethyl‐7‐methyl‐7,8‐dihydropterin, 6‐ethyl‐7‐methylpterin and a third compound, named methaniline which was identified as 4‐[2′, 3′,4′,5′‐tetrahydroxypent‐1′‐yl(5′→ 1″)O‐α‐ribofuranosyl‐5″‐phosphoric acid]aniline, in which the phosphate group is esterified with α‐hydroxyglutaric acid. Methanosarcina barkeri contains a closely related coenzyme called sarcinapterin, which was identified as a l‐glutamyl derivative of methanopterin, where the glutamate moiety is attached to the α‐carboxylic acid group of the α‐hydroxyglutaric acid moiety of methanopterin via an amide linkage.