We report contrasting electrochemical behavior of irreversibly adsorbed Sb on Pt(100) and Pt(111) single crystal electrodes. The irreversibly adsorbed Sb on both Pt(100) and Pt(111) surfaces showed redox features. The saturation charge of the irreversibly adsorbed Sb on Pt(100) was 206 +/- 3.1 mu C cm(-2) (theta = 0.33) in 0.05 M H2SO4 solution. In the Sb2O3-saturated solution, the charge of Sb on Pt(100) was increased to be 244 +/- 8.8 mu C cm-2 (theta = 0.39), which means that additional Sb is underpotentially deposited during the reduction of the irreversibly adsorbed oxygenous Sb. On Pt(111), on the other hand, the saturation charge of the irreversibly adsorbed Sb was observed to be 275 +/- 8.7 mu C cm-2 (theta = 0.38) and the charges associated with the redox feature of the irreversible Sb adsorbate decreased during cyclic voltammetric scans, depending on the Sb concentration of the contacting solution. This particular behavior of Sb on Pt(111) is interpreted in terms of the formation of an electrochemically inactive Sb species.