Determination of a representative formal redox potential of the Fe(II)/Fe(III) redox couple in cyanhaemoglobin, at pH = 7 and related to the state in solution, was the objective of this work. It was achieved at low concentrations of the protein (5 μM) to circumvent undesired adsorption. Square-wave voltammetry instead of classical cyclic voltammetry was applied because this method is more sensitive and provides information on the formal redox potential and reversibility, even for rapid processes. We obtained E°′ = − 0.12 ± 0.01 V for cyanhaemoglobin and E°′ = − 0.10 ± 0.01 V, vs. SHE, for myoglobin in comparison. These values differ by only 20 mV because the two Fe(II)/Fe(III) redox centres are embedded in closely resembling chemical environments. The small difference is probably owed to the additional axially coordinating cyanide ligand in cyanmethaemoglobin which slightly favours the Fe(III) state in the haem macrocycle.