Adlayers of Sb species on Au(100) were investigated in 0.1 M HClO4. Using in situ scanning tunneling microscopy, it was found that irreversibly adsorbed oxygenous Sb(III) species formed a quasi (2 x 2) structure on Au(100). These species underwent redox reactions on the surface without desorption. In an HClO4 solution Sb2O3, two adlayers of Sb, ((1 1)(-1 2)) and ((2 1)(-2 2)) were found. Comparison between the voltammograms recorded in HClO4 + Sb2O3 with and without H2O2 showed that the H2O2 reduction reaction is not enhanced on the adlayer of irreversibly adsorbed oxygenous Sb(III) species but it can be enhanced on the submonolayer of Sb(0).