The potential-dependent ordered atomic structures formed for iodide adsorption on Au(100) from dilute aqueous alkali metal iodide electrolytes as discerned by means of in-situ scanning tunneling microscopy (STM) are reported and compared to the corresponding behavior of the other two low-index faces, Au(111) and (110). As in related studies, emphasis is placed on linking the microscopic structural information to the voltammetric and other macroscopic electrochemical response, including the use of "potentiodynamic" STM tactics where rear-space images are acquired during appropriate electrode potential steps or sweeps. While relatively low iodide coverages, theta(I) similar to 0.1, are sufficient to lift the hexagonal Au(100) reconstruction, yielding large ordered (1 x 1) domains by ca. -0.6 V vs SCE, ordered iodide adlayers are formed only above -0.2 V. A compressible incommensurate (2 root 2 X p root 2) phase (theta(I) approximate to 0.46-0.49) is observed between ca. -0.2 and 0 V, which reverts to a distinct (2 root 2 X root 2)R45 degrees structure (theta(I) = 0.5) featuring uniform binding in 2-fold bridging sites. Interestingly, while the latter structure is retained locally at higher potentials, it is interspersed with narrow (3-4 atom wide) strips having a rotated hexagonal pattern. The latter becomes increasingly dense, and eventually dominant, toward higher potentials. This microscopically nonuniform (or "spatially modulated") phase is noteworthy as well as unusual; the corresponding iodine phases on Au(111) and (110) display, as is common, structural uniformity at a given potential. A related distinction concerns the potential-induced phase transitions : while they display sharp first-order behavior on Au(111) and (110), the phase transformations on Au(100) are diffuse. These microscopic-level differences in the adlayer phase structure and dynamics as seen by STM are also reflected in the macroscopic behavior as discerned by cyclic voltammetry and ac impedence measurements. Nevertheless, closely similar surface concentration-potential (T-E) behavior is seen for the ordered adlayers on all three low-index gold surfaces. The various adsorbate-adsorbate interatomic forces responsible for the observed potential-induced adlayer compressibilities are briefly outlined.