The rotating ring-disk electrode technique has been used to study the adsorption of Tl(I) and Tl(0) on gold in slightly acidified 0.5 M KCl solution. The processes which occur, starting at the most anodic potential sustainable without appreciable supporting electrolyte (and gold electrode) oxidation are: +0.7 V (vs. SCE) to −0.2 V, adsorption of an ionic thallium species containing a formal oxidation charge of +1, Tl(I) ads; E< −0.2 V, (1) reduction of Tl(I) ads to underpotentially deposited Tl(0), (2) reduction of Tl(I) in solution to underpotentially deposited Tl(0). The surface coverage of Tl(0) at the Nernst potential is 150 μC per real cm 2. As the electrode potential is scanned to anodic potentials, underpotentially deposited Tl(0) is oxidized to Tl(I), part of which dissolves in solution, and part of which remains as Tl(I) ads. In 0.11 m M Tl(I) solutions, the maximum coverage for Tl(I) ads is about 50 μC per real cm 2.