A new ageing mode for positive electrode, associated to Ni2O3H formation, was observed for the first time in alkaline batteries (Ni-Cd, Ni-MH) during harsh shallow cycling operations. The study of Ni2O3H synthesis conditions outlined that gamma-NiOOH and beta III-NiOOH phases display instability versus electrolyte at elevated temperature, leading to the irreversible formation of Ni2O3H through a reduction, followed by a dissolution / precipitation mechanism. It was shown also that Ni2O3H is a rather moderate electronic conductor and is almost electrochemically inactive, in good correlation with the loss of cell capacity. Highlighting NiOOH / Ni2O3H transformation provides a new understanding of nickel hydroxide and oxyhydroxide phase transformations diagram. (C) 2018 The Electrochemical Society.