The photoactivation process of a photosensitive nitrile hydratase (NHase) from Rhodococcus sp. N-771 has been investigated by 57Fe Mössbauer spectroscopy and magnetic susceptibility measurements in order to clarify the behavior of iron atoms in the enzyme. Mössbauer spectra of inactive NHase gave two symmetric-doublet components indicating the presence of two iron species, while that of the active NHase gave a single symmetric doublet indicating the presence of a single iron species. Magnetic susceptibility measurements of the inactive and active HNase both showed small effective magnetic moments. These results led us to conclude that one of the two iron atoms incorporated in the NHase is oxidized during photoactivation, namely from a low spin ferrous to a low spin ferric state. This is the first observation of an intramolecular photooxidation phenomena involving iron in a single protein molecule.