A thorough study concerning the formation of LiNiO2by a low temperature exchange reaction under hydrothermal conditions has been made. LiNiO2with low degree of cationic mixing and good electrochemical performances was prepared from NiOOH. LiCoO2is obtained from CoOOH using the same procedure. The synthesized products (LT-LiNiO2) present electrochemical properties similar to those of LiNiO2prepared by conventional powder synthesis (HT-LiNiO2) in spite of the difference in BET surface area: 20 m2/g for LT-LiNiO2and 0.7 m2/g for HT-LiNiO2. We found the thermal and moisture stability of our LT-LiNiO2prepared material to be much lower than that of HT made material. Furthermore, this instability was found to be 1) intrinsic to the material and not due to the synthetic method 2) enhanced by decreasing the LiNiO2particle size, and 3) much lower than that of LiCoO2. A complete mechanism is proposed to describe the correlation between the stability of the Ni based lithiated oxides in reducing conditions and their stability over moisture. Finally, in view of electrochemical applications, cobalt substituted products should be used in order to ensure the stability both against reduction-delithiation and hydrolysis.