The adsorption of iodine on platinum single crystals was studied with the scanning tunneling microscope (STM) to define the limits of resolution that can be obtained while imaging in air and to set a target resolution for STM imaging of metal surfaces immersed in an electrochemical cell. Two iodine adlattice unit cells of slightly different iodine packing density were clearly imaged: (√7×√7) R19.1°-I, surface coverage θI=3/ 7; and (3×3)-I, θI = 4/9. The three iodine atoms in the (√7×√7) unit cell form a regular hexagonal lattice interatomic distance dI = 0.424 nanometer, with two atoms adsorbed in threefold hollow sites and one atom adsorbed at an atop site. The (3×3) unit cell showed two different packing arrangements of the four iodine atoms exit. In one of the (3×3) structures, the iodine atoms pack to form a hexagonal lattice, dI = 0.417 nanometer, with three of the iodine atoms at two fold adsorption sites and one atom at an atop site. Another packing arrangement of iodine into the (3×3) unit cell was imaged in which the iodine atoms are not arranged symmetrically.