The stability of the Na0.6CoO2 phase, as intended to be used as a conductive additive at the positive electrode of Ni-MH batteries, was tested in electrochemical cycling conditions, in KOH alkaline electrolyte. Simple aging of Na0.6CoO2 in the electrolyte entails its spontaneous transformation into a mixture of gamma and beta(III) cobalt oxyhydroxides. This reaction, occurring within the solid state, is based on an original mechanism, which involves cationic exchange processes and passes through an intermediate interstratified phase. Electrochemical oxidation of Na0.6CoO2 in the alkaline electrolyte, as well as chemical oxidation, leads to a gamma cobalt oxyhydroxide, which can be totally reduced to beta(III), depending on the reduction conditions. It is important to understand such an evolution mechanism of the Na0.6CoO2 additive because it will have a significant and positive influence on the electrochemical cycling of the positive electrode in specific deep discharge or low potential storage conditions.