In situ scanning tunneling microscopy (STM) of the dissolution of a Cu(100) electrode in aqueous chloride media (HCl) was employed to find the mechanism of reaction at the atomic level. Imaging the surface at potentials where no dissolution occurred resulted in a clear (root 2 x root 2)R45 degrees Cl structure with an observed Cl-Cl distance of 0.35 +/- 0.02 nm, in agreement with previous ultrahigh vaccum surface analytical lower energy electron diffraction and Auger electron analysis studies of Cu(100) surfaces dosed with chlorine or aqueous Cl-. Images acquired at potentials where dissolution was modest showed the preferred reaction sites were step edges with edges always retreating along the Cu(100) {100} direction. At still higher potentials, the rate increased and the reaction sites were still mainly the step edges. This work compares well with a previous STM study of Cu(111) electrodes under similar conditions, where the dissolution of the Cu(lll) caused reaction along {211} step edges, which are very similar to the Cu(100) {100} step edges.