Irreversibly adsorbed arsenic adlayers on Pt(111) electrodes have been studied at full blockage. The voltammetric profiles for 0.1M HClO4, 0.05 M H2SO4, and 0.1M HClO4 + 0.01M sodium acetate solutions, present only small differences, indicating minimal anion effects on the arsenic surface redox process. Significant contributions to the voltammetric charge due to anion adsorption are highly unlikely, as suggested by the almost negligible amounts of adsorbed acetate anions on the arsenic-covered surface found by in situ FTIRS. A Pt(111)-(root 3x root 3)R30 degrees-As adlayer structure has been imaged by STM on emersed samples. This structure, with an ideal coverage of 0.33 As/Pt, together with the voltammetric charge density arising from the surface redox process of arsenic adsorbate, around 240 mu C cm(-2), supports a stoichiometry involving three electrons per As adatom, which blocks three platinum adsorption sites.