When microcrystalline Prussian blue particles, which are mechanically immobilised on the surface of a graphite electrode, are cyclically oxidised and reduced in contact with an electrolyte solution containing rubidium or thallium ions, it will be observed that the voltammetric signal of the hexacyanoferrate system vanishes whereas the signal of the nitrogen-coordinated iron remains active. Fully reduced Prussian blue, i.e. Everitt’s salt, contains either two rubidium or thallium ions per two iron ions. One of the Rb+ and Tl+ ions is irreversibly bonded and cannot leave the Prussian blue lattice. This prevents an oxidation of Prussian blue to Prussian yellow, The reason for the irreversible bonding of one of these cations is a lattice contraction which occurs when Everitt’s salt is oxidised to Prussian blue. In the Prussian blue state, the Tl+ and Rb+ ions cannot diffuse through the channels, which is necessary for a further oxidation, In the fully oxidised state, the so-called Prussian yellow, the channels are again widened, but this compound is not an ion conductor and thus the oxidation cannot proceed either. When thallium ions have been inserted into the cavities of Prussian blue, they can leave the cavities at very negative potentials (E(p) = -1.36 V vs. Ag/AgCl) to form metallic thallium on the graphite. For charge compensation, they are substituted in the Everitt’s salt by potassium ions, After the oxidation of the metallic thallium at -0.67 V, the thallium ions enter the Everitt’s salt lattice again and the potassium ions leave it, because thallium ion insertion is favoured by 160 mV (corresponding to a Delta G = -12.35 kJ mol(-1)). The behaviour of rubidium and thallium ions in Prussian blue can be explained with the help of a consistent geometric model of the lattice contractions and expansions which accompany the electrochemical reactions. This model is supported by independent X-ray studies.