Infrared spectra of a carbon monoxy-bound form of the EPR silent Ni(II) species of hydrogenase isolated from Chromatium vinosum are presented. These spectra show a band at 2060 cm-1 due to v(CO) for a metal-CO complex. This absorbance shifts to 2017 cm-1 upon exposure of the enzyme to 13CO. This band is attributed to v(CO) from a Ni(II)-CO species. It is shown that the CO on this species is photolabile at cryogenic temperatures but rebinds to form the original carbon monoxy species at temperatures above 200 K. In addition to the v(CO) band, infrared lines are detected at 2082, 2069, and 1929 cm-1, which shift slightly higher in frequency upon photolysis of the CO from the Ni. These infrared bands do not arise from CO itself on the basis of the fact that the frequency of these bands is unaffected by exposure of the enzyme to 13CO. Experiments in D2O show that these bands do not arise from an exchangeable hydrogen species. It is concluded that these non-CO bands arise from species near or coordinated to the Ni active site. The possible nature of these bands is discussed.