Equilibrium unfolding experiments on several mutant forms of horse heart cytochrome c were performed. By means of absorbance spectroscopy, the accumulation of an equilibrium intermediate was revealed upon unfolding of Y97V mutant protein, and its structural properties were characterized. The data obtained allow one to conclude that the equilibrium intermediate corresponds to the earliest kinetic intermediate Ic in cytochrome c folding reaction. A comparative analysis of spectral properties of unfolded states of cytochrome c induced by urea or guanidine hydrochloride is presented.