In situ scanning tunneling microscopy (STM) of the dissolution of a Cu(111) electrode in aqueous chloride media (HCl and KCl) was employed to ascertain the mechanism of the reaction at the atomic level. At potentials where there was no dissolution, atomic imaging of the Cu(111) revealed a (6 root 3 x 6 root 3)R30 degrees structure consisting of a Cl adlattice with a Cl-Cl distance of 0.39 +/- 0.02 nm which is in agreement with vacuum surface analytical (low-energy electron diffraction, Anger electron analysis and STM) studies of Cu(111) dosed with chlorine and aqueous Cl-. Imaging at potentials where the anodic dissolution of Cu begins reveals that the preferred reaction sites were step edges, and the retreating edges always ran along steps in the {211} direction. Rates of dissolution increased as the potential was increased, with the predominant reaction site being step edges. The dissolution rate was higher in the presence of K+.