Initial layers of Zn, Ni, and Zn-Ni alloy electrodeposition on Au electrode in sulfate baths are detected in a nm thickness range by in situ ellipsometry. The thin layer deposits are also analyzed by Auger electron spectroscopy (AES), and ICP atomic emission spectroscopy. The preferential deposition of Zn, in the initial thin layer of Zn-Ni alloy deposition, is confirmed by the analysis. From the ellipsometric measurements : the underpotential deposition (UPD) of Zn is observed; i.e. the Zn deposition starts at a potential more positive by 0.5 V than the redox potential of Zn2+/Zn, while noticeable UPD of Ni cannot be observed. For Zn-Ni alloy deposition, the thin layer of Zn formed in the UPD potential region greatly inhibits the Ni deposition. It is speculated that the preferential Zn deposition in Zn-Ni alloy electroplating is caused by the inhibition of Ni nuclei growth due to a thin layer of Zn on the deposited Zn-Ni alloy.