Using a home-built scanning tunnelling microscope (STM), the structure of iodine adlayers in air and in electrolyte has been investigated. In air, the occurrence of tip-adatom interactions was demonstrated and an antiphase domain boundary was resolved atomically. The adlayer structure on the Pt(100) surface in 10–4mol dm–3 I– solution (pH 4) is potential dependent. At 0.75 V vs. RHE the Pt(100)([graphic omitted] × 5[graphic omitted])R45°–I adlattice is found. At 0.5 V, domains of this structure and of a Pt(100)([graphic omitted] × [graphic omitted])R45°–I adlattice exist. Desorption of iodine (at 0.25 V) leads to a hexagonally reconstructed surface. During deposition of small amounts of copper onto the iodine-covered surface, islands with diameters between 10 and 30 Å, which are one to three atomic layers high, are formed first and then coalesce to a hill-like structure.