Electrodeposition of Hg at a well-ordered Ir(1 1 1) electrode has been examined by linear sweep voltammetry (LSV) and in situ scanning tunneling microscopy (STM) in aqueous 0.1 M HClO4 containing dilute Hg(ClO4)(2). The results of LSV revealed multiple peaks. presumably due to underpotential deposition and diffusion-limited bulk deposition. In situ STM unveiled the details of bulk Hg deposition at bare and iodine-modified Ir(1 1 1). The compositions of the Ir(1 1 1) surface could dominate the morphology of the Hg film. An iodine adlayer rendered layer-by-layer deposition of Hg till the sixth layer when 3D island growth prevailed. High-quality STM atomic resolution identified a well-ordered iodine overlayer residing on top of the Hg film, strongly suggesting that the Hg film was crystalline. At a bare Ir(1 1 1), Hg deposition was also layer-by-layer but only to a thickness of three layers. Without the iodine adlayer. the Hg film was, however, seriously pitted and it was not possible to achieve atomic resolution. (C) 2003 Elsevier B.V. All rights reserved.