Many articles have shown that polyethylene glycol (PEG), when combined with chloride ions, is a strong suppressor of copper overpotential deposition (OPD) on a copper substrate. However, few articles have explored the roles of PEG and chloride ions in copper underpotential deposition (UPD) when the cathodic substrate was polycrystalline gold. The individual roles and the interactions of PEG and chloride ions during Cu UPD on a polycrystalline gold electrode were characterized using cyclic voltammetry (CV). According to the CV patterns, a small amount of chloride ions strongly facilitated the Cu UPD at a more positive potential; PEG alone exhibited a similar CV pattern to that of the additive-free case. PEG significantly promoted the accelerating effect of chloride ions on Cu UPD, a result that is the opposite of what occurred during Cu OPD. A mechanism by which PEG-Cl accelerates Cu UPD was proposed. This mechanism also indirectly suggests that the suppressor of Cu OPD should be PEG-Cu(+)-Cl(vertical bar vertical bar), which could not form during Cu UPD on the polycrystalline gold electrode.