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A long-term cyclic voltammetry study of Pt(111) electrode in dilute solutions of mercury sulfate (5 × 10–8–5 × 10–7 M Hg2SO4 + 0.5 M H2SO4) has shown that a slow transformation of Pt(111) surface takes place. This transformation leads to a decrease in the bi-dimensional long-range order of the surface. The interpretation of the process involves the increase in mobility of Pt atoms and surface alloying in the presence of mercury. Similar processes of Pt(111) surface disordering take place in acid solution of copper sulfate with the addition of Hg2SO4. The penetration of Hg atoms beneath the Pt(111) topmost layer proceeds when only a fraction of the mercury monolayer is deposited on the electrode surface.