The Cd electrodeposition on Au(100) in sulfuric acid solution was investigated by means of in situ electrochemical atomic force microscopy (EC-AFM) and long time polarization experiments. Initially, the Cd adlayer was formed at underpotentials (E = 0.05 V/NHE) in a (root 2 x root 2)R45 degrees-Cd structure, which changed to a (1 x 1)-Cd structure at E = -0.45 V/NHE. Within the potential range from -0.3 to -0.45 V/NHE, a quick-alloying process was observed at the interface. The dynamic of the surface alloying was found to depend on polarization conditions. EC-AFM observation of the stripping process showed that the removing of the Cd bulk phase and the surface alloyed phase roughened the electrode surface. The mechanism that was proposed for surface alloying took into consideration the turnover process between Au and Cd atoms and the interdiffusion process of either Au or Cd atoms through the vacancy-rich surface alloy within the top layers. During bulk deposition at E = -0.49 V, Cd film was formed by a Stranski-Krastanov mechanism, with the Cd(0001) plane parallel to Au(100).