The rotating ring-disk electrode technique with a Pt(111) single crystal in the disk position is used to establish, for the first time, both the bromide adsorption isotherm and its electrosorption valency on Pt(111). In 0.1 M HClO4 with approximate to 10(-4) M Br-, bromide adsorption begins at ca. -0.15 V (SCE) and reaches a coverage of approximate to 0.42 ML at 0.5 V; the electrosorption valency is essentially unity. By subtracting the bromide flux from the Pt(111)-disk current, we were able to deconvolute hydrogen adsorption/desorption processes in the presence of bromide. The major peak in the voltammetry at ca. -0.1 V is the simultaneous desorption/adsorption of hydrogen and adsorption/desorption of bromide. Furthermore, we demonstrated that the saturation coverage of adsorbed hydrogen on Pt(111) is approximate to 160 mu C/cm(2).