The transformation from underpotential deposition to overpotential deposition involves, as a rule, a transition from 2D layer-by-layer deposition to 3D bulk deposition. As the reversible potential for silver deposition on platinum is approached, the amount of silver deposited exceeds a monolayer, and may in fact be close to that corresponding to three atomic layers. Some of this silver is irreversibly adsorbed and becomes part of the substrate, forming a silver-modified platinum electrode. This layer is not removed, even when the solution is replaced by the supporting electrolyte alone, with no silver ions. The amount of Ag which is irreversibly adsorbed depends on the potential of deposition, and may reach as much as 1.8 monolayers. As the potential of the silver-modified electrode is changed from the double-layer region on Pt to the region of adsorption of atomic hydrogen and back, in a solution free of Ag+ ions, a reversible transition is observed, from a smooth 2D deposit to 3D clusters. This change is clearly seen in spectroelectrochemical experiments, although the evidence obtained from anodic stripping is not conclusive. Formation of the clusters (without changing the total amount of Ag on the surface) releases some surface sites for adsorption of atomic hydrogen. This is probably the thermodynamic driving force for the transformation observed.