Electrochemical deposition (electrodeposition) and dissolution processes of Pd on a polycrystalline An electrode surface in HClO4 solution containing PdCl42- were investigated by second harmonic generation (SHG) measurements. Both 1064 nm (near-IR) and 600 nm (visible) excitations were employed to monitor the SHG behavior in situ. Under the near-IR excitation, the resulting SHG signal contained contributions from both the Pd deposit and the adsorbed PdCl42-. On the other hand, the adsorption of the PdCl42- did not affect the SHG signal under the visible excitation. The SHG signal under the visible excitation was resonantly enhanced with the first and second monolayer depositions of Pd but declined with higher Pd coverage. These results confirm the unique structure and property of an ultra-thin Pd overlayer on gold, suggested by scanning tunneling microscopy (STM), surface X-ray scattering (SXS) and electrochemical measurements at electrodeposited Pd thin layers on single crystalline gold surfaces.