Fourier transform infrared (FTIR) amide I spectroscopy has not been widely used as a method to study protein folding. Some thorough studies of thermal unfolding have been carried out; however, protein unfolding in the presence of the widely used denaturants guanidine hydrochloride and urea has only recently been reported. Guanidine hydrochloride and urea both absorb strongly in the amide I region, as does H2O. Here, we have used deuterated 13C-urea as the chemical denaturant and monitored the unfolding transition with deuterium-exchanged staphylococcal nuclease (SNase) in D2O. These conditions circumvent all subtraction difficulties as the absorption bands of D2O and denaturant are shifted out of the amide I' region [Fabian, H., and Manstch, H. H. (1995) Biochemistry 34, 13651-13655]. A very reproducible unfolding transition is obtained for SNase. 13C-Urea-induced unfolding of SNase was found not only to be comparable to previous FTIR thermal unfolding data but also to have a denatured-state spectrum similar to those of other thermally denatured proteins. The unfolding is approximately two-state. The infrared spectra in the denatured state show evidence of some residual β- sheet structure as well as other band components not attributable to random structure.