A uniform ZnO thin film was cathodically deposited on indium-tin oxide electrode and the film formation mechanism was investigated. Fourier transform infrared spectra and in-situ mass changes by electrochemical quartz crystal microbalance demonstrated that soluble intermediates were formed in the initial stage of ZnO nucleation. Time resolved analysis of morphologies and mass changes in the film formation suggested a critical pH attainment for the nucleation of zinc oxide and the precipitation-dissolution mechanism in the film growth.