In-situ scanning tunneling microscopy (STM) and ex-situ low-energy electron diffraction (LEED) have been employed to characterize an iodine-coated Au(111) electrode surface in perchloric acid solution in the absence of iodide ions. Iodine forms a well-ordered monolayer with two distinguished sets of incommensurate lattices (phases) : a centered rectangular c(p x root 3R-30 degrees) phase and a "rotated hexagonal" ("rot-hex") phase in the range of electrode potentials between 0.3 and 1.4 V vs a reversible hydrogen electrode (RHE). Both lattices become more compressed with increasing electrode potential. The (root 3 x root 3)R30 degrees phase was found to have the most open structure for the iodine monolayer on the Au(111) surface. Uniaxial compression of the c(p x root 3R-30 degrees) phase (p decreases from 3 to ca. 2.5) resulting from an increase in electrode potential was successfully observed by LEED. In the range of the electrode potential between 0.5 and 1.2 V vs RHE, LEED and in-situ STM measurements indicate a small change in the p value around 2.5. Isometrical electrocompression of the rot-hex-I phase was successfully monitored using in-situ STM by following changes in Moire patterns resulting fi om an increase in electrode potential. A detailed characterization of the rot-hex-I phase was achieved with the aid of computer-simulated STM images. The reversible phase transition between c(p x root 3R-30 degrees) and rot-hex occurred at potentials more positive than ca. 1.3 V vs RHE. Atomic size vacancies were observed at the early stages of electrooxidation of the rot-hex-iodine adlayer.