In situ scanning tunneling microscopy (STM) was used to examine multilayer Hg film electrodeposited on a well-ordered Ir(111) single crystal electrode in 0.1 M HClO4 + 1 mM Hg(ClO4)(2). Topography STM scans showed that the Ir(111) - supported Hg film electrode contained well-defined terraces separated by monatomic steps (Delta z = 2.3 angstrom). The STM's tip could be used to induce local dissolution of the Hg deposit under proper operating conditions and the depth of the etched pit informed directly the thickness of Hg deposit. Although in situ STM imaging with a tungsten tip could not result in atomic structure of bare Hg film in 0.1 M HClO4, it discerned highly ordered iodine adlayers, represented by a (2 x 8 root 3)rect - 1 structure, on the Hg film in solution containing potassium iodide. These STM results suggested that the Hg substrate could have an ordered atomic structure. (C) 2008 Elsevier B.V. All rights reserved.