In situ scanning tunneling microscopy has been employed to determine the structure of iodine adlayers formed on Ag(100) and to evaluate the role of such adlayers in the dissolution and deposition reactions of the substrate in perchloric acid. In the double-layer potential region, iodine was found to possess a c(2 x 2) structure; this structure was unaffected by anodic dissolution of the Ag substrate. On both bare and I-pretreated Ag(100) surfaces, the anodic dissolution of Ag occurred exclusively at step-edges in a layer-by-layer mechanism. However, whereas jagged monatomic step lines were found on the I-free surface, relatively smooth (straight) step lines existed along the direction of iodine atomic rows ({100}) on the Ag(100)-I surface. Etching rates on Ag(100)-I were dependent on the direction of the step-edges, which suggests that the adlayer near such edges plays a crucial role in the dissolution reaction. The deposition of Ag on Ag(100)-I was also found to occur preferentially at step-edges aligned along a particular direction. A model is proposed to explain the anisotropic dissolution and deposition.