Abstract The a.c. impedance spectra have been determined, across the nickel/cobalt hydroxide range, for thin film electrodes. The electrodes were prepared by cathodic deposition and spectra were obtained under various conditions of charge and discharge, by application of sequential d.c. potential steps. Estimates of the electrode surface area showed that the thin film microporosity is increased by the addition of cobalt, reaching a maximum at cobalt levels greater than 15%. The surface area results have been related to the electrode performance and the extension of the redox potential region, with increasing cobalt content, has been confirmed by the spectra observed.