The electrochemistry of hydrated nickel oxides (NiOxHy) has been an active field of research , principally due to the use of these materials as the positive electrode in secondary nickel /cadmium batteries. Sintered plate materials, as well as electrochemically and chemically grown thin films have been studied; the published literature is reviewed in refs. and However, and in spite of the great effort in this direction, some important aspects of the electrochemical behaviour of these materials remain still unclear. Among them, one could emphasize the mechanism of the electrochemical reaction as a whole, the mechanism by which diffusion of the ions into the film occurs, the role of water molecules incorporated in the film structure, and the dependence of the electrochemical properties on the film microstructure.