The characteristics of hydrous nickel oxide layers formed by the application of a fast periodic square-wave potential routine to a polycrystalline nickel electrode in alkaline solutions of different composition have been studied. The charging/discharging properties of the electroformed oxide layers were derived from voltammetry. Infrared spectroscopy data indicated that the nickel oxide layers resulting from vacuum drying at 100 degrees C consisted of a mixture of Ni(II) and Ni(III) oxides, the Ni(II)/Ni(III) ratio depending on the characteristics of the potential routine. X-ray analysis data and SEM micrographs showed the amorphous nature of the thick hydrous nickel hydroxide films produced by the above mentioned procedure. A correlation between the properties of the nickel oxide layers and their electrochemical response in alkaline solution has been advanced.