Fe^III mimochrome I is the prototype of a new class of hemoprotein models characterized by a covalent helix–heme–helix sandwich. It contains deuterohemin bound through two propionyl groups to two identical N‐ and C‐terminal protected α‐helical nonapeptides, each of which bears a His residue (a potential axial lig‐and of the iron ion) in the central position. In order to understand better the three‐dimensional structure of Fe^III mimochrome I and its correlation with spectral properties, we have characterized the fully diamagnetic parent compound Co^III mimochrome I by UV/visible, CD, and NMR spectroscopy, coupled with conformational energy calculations. Co^III mimochrome I is a highly water‐soluble compound present in solution as two isomers, which slowly interconvert only at very low pH values. These isomers were isolated and separately characterized. Their UV/visible spectral properties are very similar, while their CD spectral properties differ markedly in both the far UV and Soret regions. The isomers were identified by ¹H NMR spectroscopy as diastereomers of the δ and δ type. This is the first example of an accurate three‐dimensional structure determination in solution of a hemoprotein mimetic that allows a straightforward correlation between structure and spectral properties.