We describe an approach that leads to a mechanism describing the lead ion adsorption and lead underpotential deposition (UPD) on a polycrystalline gold electrode in supporting electrolytes containing chloride, nitrate and/or perchlorate. By combining the electrochemical quartz crystal microbalance (EQCM) method with data from X-ray photoelectron spectroscopy and time of flight secondary ion mass spectrometry, the existence of adsorbed anionic ligands was demonstrated. The lead UPD process goes through the reduction of both an adsorbed lead complex and a lead species present in the supporting electrolyte.