AbstractThe myoglobin (Mb) heme Fe‐O‐N=O and heme Fe‐O‐N=O/2‐nitrovinyl species have been characterized by resonance Raman spectroscopy. In the heme Fe‐O‐N=O species, the bound nitrite ligand is removed by solvent exchange, thus reforming metmyoglobin (metMb). The high‐spin heme Fe‐O‐N=O unit is converted into a low‐spin heme Fe‐O‐N=O/2‐nitrovinyl species that can be reversibly switched between a low‐ and a high‐spin state without removing the bound nitrite ligand, as observed in the case of the heme Fe‐O‐N=O species. This spin‐state change is likely to be accompanied by a general structural rearrangement in the protein‐binding pocket. This example is the first of a globin protein that can reversibly change its metal spin state through an internal perturbation. These findings provide a basis for understanding the structure–function relationship of the spin cross found in other metalloenzymes and FeIII–porphyrin complexes.