The nature of the ethyl isocyanide complexes of ferri- and ferro-heme was investigated in terms of the magnetic circular dichroism (MCD) spectroscopy. The complex formation of heme with ethyl isocyanide was critically dependent on the solvent system, thus, on structures of heme before addition of the ligand. The MCD spectra of the heme ethyl isocyanide complexes revealed that: (1) The addition of ethyl isocyanide to both ferri- and ferro-heme in an aqueous solution gave respective high-spin heme complexes with no more than one ethyl isocyanide molecule per one heme. (2) The ferro-heme with ethyl isocyanide in an aqueous ethanol solution formed a typical monomeric low spin ferro-hemochrome with two ligands (ethyl isocyanide) on both sides of the heme plane. (3) In the presence of excess ethyl isocyanide in an aqueous ethanol solution the ferri-heme gave an MCD spectrum similar to that for the monomeric low spin ferro-hemochrone. Therefore, ethyl isocyanide was believed to reduce the ferri-heme in an aqueous ethanol solution. (4) The ferro-heme ethyl isocyanide complex in an aqueous solution offered a Faraday B term around 450 nm instead of an apparent Faraday A term observed for the ethyl isocyanide complex of the cytochrome P-450. These results are not consistent with the view that the ferro-heme ethyl isocyanide complex in an aqueous solution can be a heme chromophore model for the cytochrome P-450 ethyl isocyanide complex.