After reduction of human methaemoglobin by solvated electrons a non-equilibrium low-spin state of deoxyhaemoglobin is formed which has the characteristic haemochrome spectrum. This haemochrome state is ascribed to a weakly 6-coordinated structure of the haem, which is stabilised by the protonated distal histidine. Oxygen binding is not inhibited by the presence of the weak interaction in the haemochrome state. From the pH dependence of the biphasic behaviour of the oxygen binding a pK of about 8.8 is obtained which is ascribed to the deprotonation of the distal histidine which is in the proximity of a negative ion. A model is proposed to explain the complex spin-equilibria observed in methaemoglobin. The enthalpy of activation of the decay of the haemochrome state is about 53 kJ x mol(-1) and increases to 90 kJ x mol(-1) in the presence of 1 M methanol, indicating a strong interaction between methanol and haemoglobin. Around pH 8.4 the rate constant of the binding of oxygen to the haemochrome state is so high that it may well be diffusion controlled.