The electrochemical behaviour of hexacyanoferrates containing different ratios of iron and nickel, attached mechanically to a graphite electrode, has been studied using cyclic voltammetric techniques. The results have shown that mixed crystals of Fe-Ni hexacyanoferrates are formed, with the nitrogen-coordinated iron being replaced by nickel in the lattice as the percentage of nickel increases, whilst retaining the lattice symmetry. X-ray diffraction data confirmed this theory.