Reported here are in situ scanning tunneling microscopy (STM) results of underpotential deposition (UPD) of copper at an iodine-modified Pt(100) electrode, The cyclic voltammograms reveal that an iodine adlayer strongly adsorbed on Pt(100) resulted in a 350 mV delay of Cu deposition with respect to that of a bare Pt(100) electrode. High-quality in situ STM imaging reveals the atomic structures of the iodine adlayers before and after the deposition of Cu adatoms, Depending on the dosage of iodine vapor to an annealed Pt(100) electrode, two ordered structures of the iodine adatoms are formed and are characterized as (root2 x 5 root2)R45 degrees and (root2 x 9 root2)R45 degrees with coverage of 0.6 and 0.55, respectively. The predominant interadsorbate interaction results in adsorption of iodine at either symmetric it-fold or asymmetric sites. Since the Cu deposit displaces the iodine adatoms on Pt(100), the iodine adatoms with weaker surface bonding are replaced first, leading to anisotropic deposition. Regardless of the initial iodine structures on Pt(100), deposition of Cu results in a c(2 x 2) structure. The deposition and dissolution processes are reversible with respect to the Cu adatoms but the iodine overlayer undergoes irreversible reconstruction from (root2 x 5 root2)R45 degrees to c(2 x 2), In situ STM atomic imaging at the onset of bulk Cu deposition also reveals protruding islands with ordered c(2 x 2) structure of iodine adatoms, suggesting a pseudomorphic structure of the 2nd Cu adlayer.