The electronic structure (charge distribution, bond indices) and the geometry (bond distances and angles) of the deoxyheme and the oxyheme with coordinated proximal histidine in their reduced and oxidized form were determined by the INDO method. The effect of the distal histidine (in the case of the oxyheme) and a water molecule (in the case of the metheme) on the geometry, charge distribution and stability of the systems was investigated. The method was adopted to model the oxidation of myoglobin in biological systems. The results revealed that both deoxy- and oxymyoglobin could spontaneously undergo one-electron oxidation. The mechanistic considerations based on the charge distribution and energetic effects led to the conclusion, that in oxymyoglobin’s case the electron transfer are followed by dissociation of a dioxygen molecule and addition of a water molecule, where both processes proceed in parallel.