Postsynthesis thermal treatments at various temperatures in air have been applied to LiNiO₂, and the induced structural and electronic changes have been uncovered. Except for the familiar decomposition process at higher temperatures, a series of transformations also take place under mild conditions. To identify such subtle changes, ex situ and in situ synchrotron radiation diffraction, ex situ ⁷Li nuclear magnetic resonance spectroscopy, and ex situ measurements of magnetic properties have been performed. We show that the reaction between LiNiO₂ and CO₂ starts already at a temperature of 200 °C, forming Li_1-zNi_1+zO₂ layers. If the thickness of this layer is well adjusted, the electrochemical performance of LiNO₂ can be improved. A cation off-stoichiometry of [Li_0.90Ni_0.10]NiO₂ is identified at 600 °C even before the decomposition occurs. We also investigate the interplay of the reaction between LiNiO₂ and CO₂ with the decomposition at 700 °C. The changes in the Ni oxidation state and local Li environments are also monitored during the whole process.