Carbon monoxide dehydrogenase from Methanosarcina barkeri, purified to 95% homogeneity, contains 30 Fe, 2 Ni, 1 Zn, and 1 Cu (per alpha 2 beta 2 enzyme). Core extrusion experiments indicate 6 [4Fe-4S] clusters/tetramer, and electron paramagnetic resonance (epr) spectroscopy detects at least one of these clusters, in the reduced form, with apparent g values of 2.05, 1.94, and 1.90, and Em9.2-390 mV. A second epr signal, also seen in the reduced enzyme, has apparent g values of 2.005, 1.91, and 1.76, and Em9.2-35 mV. Two signals were seen in thionin-oxidized enzyme, one with a line shape suggestive of Cu(II), and the other resembling that of a [3Fe-4S] cluster. The enzymes nonphysiological substrate, CO, caused several spectral changes to the reduced enzyme, most notably a shift of the g = 1.76 feature to g = 1.73.