The multienzyme carbon monoxide dehydrogenase complex from Methanosarcina thermophila contains at least two protein components: a CO-oxidizing nickel/iron-sulfur (Ni/Fe-S) component and a cobalt-containing corrinoid/iron-sulfur component (Co/Fe-S). The CO dehydrogenase complex has been shown to synthesize acetyl-CoA from CoA, CH3I, and CO as well as to cleave acetyl-CoA into its methyl, carbonyl, and CoA components as the first step in the catabolism of acetyl-CoA to methane and CO2. Presumed to serve as an acceptor of the methyl group of acetyl-CoA en route to methane, the Co/Fe-S component contains iron, acid-labile sulfur, and a corrinoid cofactor (factor III) that is the site of methylation. Using EPR spectroscopy and spectroelectrochemistry, we characterized the cobalt and Fe-S centers of the Co/Fe-S component. The redox and EPR properties of the metal centers in the isolated Co/Fe-S component are similar to those of the Co/Fe-S component in the CO dehydrogenase enzyme complex, a result that indicates that any protein-protein interaction between components in the complex has little influence on the properties of the metal centers. The corrinoid is maintained in the base-off state with a formal equilibrium reduction potential (E'o) at pH 7.8 of -486 mV for the Co2+/1+ couple that facilitates reduction of the Co2+ state by approximately 12 kcal/mol relative to base-on cobamides. The Co/Fe-S component also contains a [4Fe-4S]2+/1+ cluster with an E'o at pH 7.8 of -502 mV, which is nearly isopotential with the Co2+/1+ couple of the cobamide.