A site for the binding of exogenously added carbon monoxide has been identified at the active site of the Fe-only hydrogenase (CpI) from Clostridium pasteurianum. The binding and inhibition of carbon monoxide have been exploited in biochemical and spectroscopic studies to gain mechanistic insights. In the present study, we have taken advantage of the ability to generate an irreversibly carbon monoxide bound state of CpI. The crystallization and structural characterization of CpI inhibited in the presence of carbon monoxide indicates the addition of a single molecule of carbon monoxide. The ability to generate crystals of the carbon monoxide bound state of the hydrogenase that are isomorphous to those of the native enzyme has allowed for a direct comparison of the crystallographic data and an unambiguous identification of the site of carbon monoxide binding at the active site of CpI. Carbon monoxide binds to an Fe atom of the 2Fe subcluster at the site of a terminally bound water molecule in the as crystallized native state of CpI that has been previously suggested to be a potential site of reversible hydrogen oxidation. Binding of carbon monoxide at this site results in an active site that is coordinately saturated with strong ligands (S, CO, and CN), providing a rational potential mechanism for inhibition of reversible hydrogen oxidation at the active site of CpI.