Chemically polished Ag(100) electrodes with and without lead adsorbates and phase deposits were investigated in situ in 0.5 M NaClO4 by Scanning Tunneling Microscopy (STM), using a potentiostatic STM assembly. During the potential-controlled lead adsorption and phase deposition, the tunneling mode was maintained. On the lead-free electrode surface, parts with flat terraces of about 5-20 nm width, where monoatomic steps are resolved, are interspersed by densely terraced dome-shaped domains. Lead adsorption is accompanied by displacement of steps and smoothing of sub-nm corrugations, as well as by a steeper decay of the tunneling current with tip-substrate separation. Along certain parts of the substrate, repetitive Pb phase deposition/dissolution leads to formation of pronounced nm-scale steps on the Ag substrate. © 1989.