This study examined the aging mechanisms of layered cathode materials for lithium batteries upon exposure to air and the influence of this aging on the thermal stability and electrochemical performance of these materials composed of solid solutions of LiMO2 (M = [MnNi] or [MnNiCo]) in Li cells. A unique methodology for the quantitative characterization of surface carbonates on LiMO2 compounds based on differential scanning calorimeter (DSC) measurements was developed. Correlations were made between the formation of Li2CO3 and other carbonates on the surface of the lithiated metal oxide powders and the changes in the structure and electrochemical performance of the cathode materials. The techniques used included solid-state NMR, X-ray photelectron spectroscopy, Fourier transform IR, high resolution scanning electron microscopy, high resolution transmission electron microscopy and the thermal analysis, DSC, and accelerating rate calorimetry in conjunction with electrochemical measurements. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3474222] All rights reserved.