This work presents an electrochemical scanning tunneling microscopy study of Sb irreversibly adsorbed on Pt(111) at various potentials. At an open circuit potential (0.46 V vs a Ag/AgCl electrode), well-ordered structures of SbO+ were found: four (4 x 3)-3SbO+ structures and one (2 square root(3) x 2 square root(3))R30 degrees-3SbO+ structure. In addition, several unidentifiable transient structures of SbO+ were observed, and their relations to the well-ordered structures of (4 x 3) and (2 square root(3) x 2 square root(3))R30 degrees, regarding structural evolution, were proposed. At a reducing potential (0 V), the Pt(111) surface was covered with irreversibly adsorbed Sb which consisted of three different domains: protruded domain, domain of uniaxially incommensurate (square root(3) x square root(2))-Sb, and domain of bare (1 x 1) Pt(111). During oxidation of elemental Sb at 0.30 V, the Sb domains of the (square root(3) x square root(2)) structure were oxidized, while the protruded domains were not oxidized. After underpotential deposition of additional Sb onto the Pt(111) covered with irreversibly adsorbed Sb, the whole surface was filled with the Sb domains where each Sb atoms were separated by the square root(2a) distance (a = one Pt-Pt distance, 0.277 nm). The observed electrochemical inactivity below 0.3 V was discussed in terms of the protruded domain of a presumable incommensurate (square root(2) x square root(2)) structure.