In situ scanning tunneling microscopy (STM) has been used to examine underpotential deposition (UPD) of Cu at Pt(111) electrodes in the solutions of 0.01 M HCl and 1 mM Cu(ClO4)2. Cyclic voltammetry reveals two well-defined features at 0.72 and 0.55 V (vs RHE), where a tailing phenomenon is noted for the former peak. In situ STM imaging reveals a disorder-to-ordered transition of the adlayer, as the electrochemical potential of Pt(111) was stepped from 0.8 to 0.7 V to facilitate the deposition of a sub-monolayer of Cu adatoms. The ordered adlattice can be approximately characterized as (4 × 4), whose irregular intensity modulation of the STM atomic features indicates its incommensuratity. Toward the end of the first UPD feature, deposition of Cu continues, resulting in reconstruction of the (4 × 4) adlattice to (√7 √7)R19.1°. The latter structure is then stable toward further negative potential stepping to the end of the second UPD wave. In situ STM imaging turns fuzzy when the electrochemical potential of Pt(111) is made more negative to the Nernst value. Varying the operation parameters of the STM can result in atomic structures of not only the upper layer of chloride but also the lower metallic adlayer or the Pt(111) substrate.