Cesium adsorbed on a Pt(111) surface is studied in 0.5 M HClO₄. When the surface is extensively cleaned in UHV by Ar⁺ sputtering, adsorbed Cs completely dissolves in the solution and the cyclic voltammetry is the same as for clean Pt(111). Standard UHV cleaning procedures leave a surface that is more reactive: Cesium is detected on the surface after cycling the potential between -0.03 and 1.6 V, and the cyclic voltammogram shows a shift of the surface oxidation to more negative potentials.