Lithium nickel oxide, a potential candidate for cathode material for lithium-ion batteries, showed a distinct deterioration after storage in air for a time. The origin of this deterioration was explored by investigating surface structure, surface species, and ionic oxidation state of fresh and stored LiNiO2 materials. Rietveld analysis of X-ray diffraction patterns showed not only the formation of Li2CO3 on the surface, but also a weakening of ordered layered structure for the stored materials. X-ray photoelectron spectroscopy revealed that Ni3+ transforms to Ni2+ and active oxygen species exist on the surface of stored materials. Temperature programmed desorption-mass spectroscopy measurements gave evidence that active oxygen species (O-, O-2(-)) occur on the surface of LiNiO2 after storage. A surface reaction mechanism based on the transformation of Ni3+/Ni2+ and the evolution of active oxygen species is proposed. (C) 2004 The Electrochemical Society.