The potential-dependent ordered atomic structures formed for iodide adsorption on Au(100) from dilute aqueous alkali 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 real-space images are acquired during appropriate electrode potential steps or sweeps. While relatively low iodide coverages, theta sub I approx. 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 sq rt 2 x p sq rt 2) phase (theta sub I, approx. 0.46 - 0.49) is observed between ca -0.2 and 0 V, which reverts to a distinct (2 sq rt of 2 x sq rt of 2)R45 deg structure (theta sub I approx. 0. 5) featuring uniform binding in twofold 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, towards 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.