In situ scanning tunneling microscopy (STM) has been applied to study adsorption and oxide formation on Cu(111) in weakly alkaline, neutral, and acidic sulfate-containing solutions. In 5 x 10(-4) M NaOH + 5 x 10(-3) MNa2SO4 (pH similar to10.5), a hexagonally structured OH layer is formed at potentials well below oxide formation despite the tenfold larger sulfate concentration showing a stronger interaction of OH- with the Cu surface than SO42-. In 5 x 10(-3) MNa2SO4 (pH; 7), a partially structured and highly mobile adlayer is formed at potentials quite near the onset of oxide formation with coexisting (3 x 3)-ordered domains and nonordered domains. This is assigned to the competitive adsorption of water molecules with sulfate anions. In contrast, a highly ordered and stable (root3 x root7) adsorbed layer is formed in 5 x 10(-3) MH2SO4. It is assigned to the coadsorption of sulfate anions and cations (H3O+ or H+), the latter compensating the repulsive forces between sulfate anions and thus stabilizing the adlayer. The anodic Cu(I) oxide is (111)-oriented and has a faceted surface in solutions of alkaline, weakly alkaline, and neutral pH. (C) 2003 The Electrochemical Society.