The electrodeposition of Pb onto Au(1 1 1) from a Lewis acidic ionic liquid, 1-methyl-3-butylimidazolium chloride-aluminum chloride has been investigated by STM. In the UPD range, STM imaging shows three different processes: terrace expansion at 0.8 V, island growth at 0.73 V, and monolayer formation with triangular vacancies at 0.5 V. High-resolution STM images reveals a Moire-like pattern of Pb adatoms. Moreover, a smooth surface can be formed when potentials are changed progressively. Therefore, a quasi-equilibrium process can be expected. These results indicate that Pb UPD is diffusion-controlled. In the overpotential deposition (OPD) range, the nucleation of 3D Pb crystal occurs. (C) 2010 Elsevier B. V. All rights reserved.