Cobalt is usually post-added as CoO or Co(OH)2 to nickel hydroxide at the positive electrode (nickel oxide electrode) of alkaline batteries, to form a conductive network. In the present work, we focus on the transformation of CoO and Co(OH)2 phases when oxidized at 90°C. The Co3O4 phase is the majority product of such a reaction, with CoOOH as a secondary product. It is shown that the Co3O4 phase results from the reaction of the CoOOH phase, formed by electrochemical oxidation of Co(OH)2, with Co2+ species in the electrolyte, which is made possible by temperature. This process requires a global migration of the cobalt phases towards the current collector.