Electrochemical scanning tunneling microscopy (STM) observations have been performed on a highly ordered pyrolytic graphite (HOPG) electrode modified with a cast film of hydrotalcite (HT) crystal ([Mg6Al2(OH)16]3/4CO3·1/2Cl·2H2O). The electrode was in contact with an aqueous solution of 5 mM Na3[Fe(CN)6] and 0.1 M Na2SO4 at 600 mV (vs Ag/AgCl). The STM image of the electrode showed a two-dimensional lattice with a = 1.40 ± 0.04 nm, b = 1.85 ± 0.04 nm, and α = 63 ± 3°. One bright spot in the image was present at every 3.4 [Mg6Al2(OH)16]2+ units, leading to the conclusion that the spot represented a [Fe(CN)6]3- ion adsorbed on a crystal surface. At 81 mV (vs Ag/AgCl), the STM image exhibited a two-dimensional lattice with a = 1.74 ± 0.04 nm, b = 1.83 ± 0.04 nm and α = 60 ± 3°. This was ascribed to the lattice made by a mixture of [Fe(CN)6]3- and [Fe(CN)6]4- on a HT surface. At −200 mV (vs Ag/AgCl), the STM image showed a two-dimensional lattice with a = 1.94 ± 0.04 nm, b = 1.90 ± 0.04 nm and α = 60 ± 3°, wh...