The structure of an iodine adlayer electrochemically deposited on a gold(lll) single crystal in potassium iodide solution was investigated as a function of electrode potential by means of ex situ Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), and in situ electrochemical scanning tunneling microscopy (STM). All LEED patterns obtained after emersion from 1 mM KI indicated rectangular adlattices which are all described as c(px root 3R-30 degrees), where p continuously decreased from 3 (c(3x root 3R-30 degrees) = (root 3x root 3)R30 degrees) to 2.49 with increasing positive potential. The decrease of p indicates compression of the rectangular adlattice only in one direction. In situ STM also revealed the c(px root/3R-30 degrees) adlattices except on the positive side of the potential region, where rotated hexagonal adlattices were observed. The rotated hexagonal adlattices were analyzed in detail by simulation, which indicated isometric compression of the adlattice. These results by ex situ LEED and in situ STM were compared with previously reported data by surface X-ray scattering.