Abstract The magnetic susceptibilities of reduced and oxygenated hemerythrin solutions of the same concentration were measured by a Gouy magnetic balance at 25°. The susceptibility of the reduced solution was found to be higher than that of the oxygenated one, indicating a paramagnetic contribution of iron atoms in the reduced molecule. With the assumptions that the four groups acting as oxygen combining centers in the molecule are equivalent to one another and that they are magnetostatically independent, the number of unpaired electrons in the group was calculated to be twelve in the reduced state and zero in the oxygenated preparation. In the reduced state the bonds of the iron atoms are ionic or ion-dipole type in nature. A profound change in the electronic structure takes place when oxygen molecules are combined with the hemerythrin molecule. An oxygen molecule not only loses its own paramagnetism, but also affects all three iron atoms in the group in such a way that the bond type is essentially covalent. These iron atoms are supposed to be coupled with each other in a polynuclear complex so as to be able to transmit the effect of the change of electronic state directly to the others.