We employed in situ scanning tunneling microscopy (STM) and cyclic voltammetry to study the adsorption of iodobenzene and iodoheptane molecules onto a well-ordered Pt(111) electrode in 0.1 M HClO4. These molecules were adsorbed intact without noticeable decomposition at potentials negative of 0.9 V (vs a reversible hydrogen electrode), whereas degradation became prominent at potentials positive of 0.9 V. The coverage of the iodobenzene monolayer is estimated to be about 0.45 from the amount of charges involved in its reductive desorption in KClO4 (pH 10) solutions. This value nearly equals that of iodine atoms at Pt(111). In situ STM was used to probe the spatial arrangements of these adsorbates as a function of potential in 0.1 M HClO4.A well-packed (root7 x root7)R19.1degrees-iodobenzene structure predominated at 0.3 V, which readily rearranged into a (3 x 3) adlattice as a result of a slight increase of coverage at more positive potentials. The. STM appearances of these two adlattices resemble those of iodine atoms on Pt(111), suggesting that the contrast in STM arises mainly from the iodine headgroups. More negative potentials resulted in desorption of iodobenzene molecules and a disordered adlayer. In contrast, iodoheptane molecules were disordered at potentials negative of 0.7 V, but reorganization occurred to produce local (root7 x root7)R19.1degrees at more positive potentials. These, phase transitions were reversible to the modulation of potential between 0 and 0.9 V. The electrochemical potential dominated not only the chemical nature but also the spatial arrangements of the alkyl or, aryl iodide molecules. The tip-and-sample interaction was noticeably stronger for iodoheptane, resulting in disordering of the adlayer.