The kinetics of thin alloy film formation and growth during Cd electrodeposition on Au(100) and Ag(100) has been studied at room temperature in 50 mM H2SO4 + 1 mM CdSO4 solution within the Cd underpotential range from − 0.3 to − 0.45 V. Electrochemical experiments and atomic force microscopy studies have shown that the overall alloying process at the Cd2+/Au(100) and Cd2+/Ag(100) interfaces consisted of two processes: one very fast, which occurred within a few atomic layers and is characterized by a diffusion coefficient, D≈10−16 cm2 s−1, and another much slower, which is characterized by D≈10−19 cm2 s−1, suggesting a solid state diffusion process. Based on a new kinetic approach to the alloying process at the metal/electrolyte interface in the underpotential region, the differences in kinetic parameters are discussed.