The influence of chloride adsorption and lead upd on the step dynamics of Cu(100) has been examined in acid perchlorate solution. A c(2 x 2) Cl adlayer is formed upon immersion of a Cu(100) electrode leading to step faceting in the [100] direction. At more negative potentials an order-disorder transformation occurs leading to significant rearrangement of the steps. Alternatively, in an electrolyte containing Pb2+ th, halide adlayer may be completely displaced by lead upd. Images of orthogonal surface steps in combination with an assessment of the coulometry suggest that the lead adlayer forms either a highly defective c(2 x 2), c(5 root 2 x root 2)R45 degrees or a disordered structure corresponding to a coverage ranging from 0.5 to 0.6. The transformation from the halide to the lead adlayer results in the formation of vacancies and adatoms which condense to form holes and islands, respectively. These features may be rationalized by the formation of an alloy phase at low coverage, which subsequently dealloys as the coverage approaches 0.5. The extent of the morphological changes associated with the alloying/dealloying processes is strongly path dependent. Voltammetry reveals that the stripping of the lead upd layer is associated with two oxidation waves. As the potential is increased beyond the peak of the first wave islands disappear which may be due to alloy formation occurring coincident with partial desorption of the lead. The second wave is associated with the nucleation [100]-oriented rows of the c(2 x 2) Cl adlayer which propagate across the terraces displacing the lead phase. he use of metal upd and anions as surfactants in the electrochemical deposition of copper is likely to prove even more interesting than in vacuum deposition since the surfactant coverage and its effect on mesoscopic structure can be continuously manipulated by potential control.