The synergic effect of poly(ethylene glycol) together with chloride ion for inhibiting copper deposition in copper electroplating has been of particular interest for some time. In this study, the potential-dependent behavior of poly(ethylene glycol), with or without chloride ion in a copper electroplating bath is investigated using the surface-enhanced Raman spectroscopy technique. The presence of chloride proves to play a significant role in enhancing PEG adsorption to the Cu electrode surface. More importantly, spectroscopic evidence strongly suggests the formation of a PEG-Cu-Cl complex. By comparing experiment with vibrational modes calculated by using the Hartree-Fock method with a 3-21G* basis set, the structure of this complex is proposed to be a three-coordinated Cu center with two oxygen atoms from PEG and one chloride ligand.