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)]3/4CO(3) . 1/2Cl . 2H(2)O). The electrode was in contact with an aqueous solution of 5 mM Na-3[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 alpha = 63 +/- 3 degrees. 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 alpha = 60 +/- 3 degrees. 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 alpha = 60 +/- 3 degrees, which was ascribed to the lattice made by reduced [Fe(CN)(6)].(4-) These results confirmed that an ordered molecular adsorption layer changed dynamically during the electrochemical processes.