Infrared spectroscopy has been used to examine the oxidized and CO-inhibited forms of Fe-only hydrogenase I from Clostridium pasteurianum. For the oxidized enzyme, five bands are detected in the infrared spectral region between 2100 and 1800 cm(-1). The pattern of infrared bands is consistent with the presence of two terminally coordinated carbon monoxide molecules, two terminally coordinated cyanide molecules, and one bridging carbon monoxide molecule, ligated to the Fe atoms of the active site [2Fe] subcluster. Infrared spectra of the carbon monoxide-inhibited state, prepared using both natural abundance CO and 13CO, indicate that the two terminally coordinated CO ligands that are intrinsic to the enzyme are coordinated to different Fe atoms of the active site [2Fe] subcluster. Irradiation of the CO-inhibited state at cryogenic temperatures gives rise to two species with dramatically different infrared spectra. The first species has an infrared spectrum identical to the spectrum of the oxidized enzyme, and can be assigned as arising from the photolysis of the exogenous CO from the active site. This species, which has been observed in X-ray crystallographic measurements [Lemon, B. J., and Peters, J. W. (2000) J. Am. Chem. Soc. 122, 3793], decays above 150 K. The second light-induced species decays above 80 K and is characterized by loss of the infrared band associated with the Fe bridging CO at 1809 cm(-1). Potential models for the second photolysis event are discussed.