A morphological variation of Au(111) covered with irreversibly adsorbed Sb was investigated with cyclic voltammetry and EC-STM. At open circuit potential (approximately 0.0 V vs a Ag/AgCl reference electrode), the oxygenated Sb layers were formed as an island on the wide terraces and a terrace at the step edges of Au(111). The ultimate morphology at the open circuit potential was a network adlayer with a (√ 3 x √ 3)-R30° atomic arrangement. When the oxygenated layer was reduced, the adsorption features, such as the island, shrunk or disappeared depending on their sizes. This modification was interpreted in terms of an alloy formation of Sb and Au. All of the Sb atoms, however, were not involved in the alloy formation, although the alloyed and unalloyed domains showed (√ 3 x √ 3)R30° atomic structures with different brightness in EC-STM images. During oxidation of the reduced Sb layers, the alloyed and unalloyed domains of Sb behaved in a different way: the alloyed Sb was stripped to a soluble species to leave pits, while the unalloyed Sb became an oxygenated adspecies, which desorbed very slowly. A long oxidation led to a Au(I 11) covered with pits and islands of (1 x 1) without any adsorbed Sb.