Electrochemical atomic layer deposition (e-ALD) is an effective method for achieving atomic monolayer-by-monolayer deposition of metals. In this presentation, we will discuss the mechanistic events underlying gold (Au) e-ALD. The e-ALD process involves lead underpotential deposition (Pb_UPD) followed by spontaneous redox replacement (SLRR) of Pb_UPD by Au. During Au e-ALD, a unique three-stage mass transient is observed. Using voltammetry, electrochemical quartz crystal microgravimetry (e-QCM), and chronoamperometry, we present an in-depth study of the characteristics of this mass transient during Au e-ALD. Adsorption of Au⁺³-ligand complex(es) (Au-L) on the Au surface is shown to play a key role in the Au-ALD process. The adsorbed Au-L species are reduced in stage I while a Pb_UPD adlayer is formed. The SLRR of Pb_UPD adlayer by nobler Au occurs in stage II, followed by the re-adsorption of the Au-L species in stage III. A quantitative analysis of the deposit mass under different conditions allows estimation of the average molecular weight of the adsorbed Au-L complexes. Ramifications of such adsorption-mediated deposition to thickness uniformity, conformality and film roughness will be discussed.