The structure of the carbon dioxide reduction factor from Methanobacterium thermoautotrophicum has been assigned by analysis of the intact molecule and its hydrolytic fragments as 4-[N-(4,5,7-tricarboxyheptanoyl-γ-L-glutamyl-L-glutamyl)-p-(β-aminoethyl)phenoxymethyl]-2-(aminomethyl)furan. High-resolution NMR spectroscopy and FAB, FD, and EI mass spectrometry were used, with FABMS being decisive. The trivial name methanofuran is proposed for the carbon dioxide reduction factor.