Carbon monoxide dehydrogenase (CODH) plays a central role in a recently discovered pathway of anaerobic CO and CO{sub 2} fixation. In this communication, the EPR properties of a paramagnetic species that results from azide treatment of reduced CODH are explored. Azide, an inhibitor of CO oxidation, was found to dramatically alter the EPR spectrum of center C that is normally observed for untreated samples under reducing conditions. Electron spin echo envelope modulation (ESEEM) measurements show that azide is bound to the new paramagnetic species. 29 refs., 3 figs.