Understanding the thermal stability of major reaction products, Li2O2 (space group P6(3)/mmc) and Li2O (space group Fm (3) over barm) is critical to improve the safety characteristics of Li-air batteries. The changes in the crystal structure and surface chemistry of Li2O2 and Li2O were examined as a function of temperature via in situ X-ray diffraction (XRD) and in situ X-ray photoelectron spectroscopy (XPS). Significant decreases in the lattice parameters and the c/a ratio of Li2O2 were found at 280 degrees C and higher. These structural changes can be attributed to the transformation of Li2O2 to Li2O2-delta, which is supported by density functional theory calculations. Upon further heating to 700 degrees C, a lithium-deficient Li2-delta O phase appeared at 300 degrees C and gradually became stoichiometric upon further heating to similar to 550 degrees C. XPS measurements of Li2O2 revealed that Li2O appeared on the surface starting at 250 degrees C, which is in agreement with the onset temperature of phase transformation as detected by XRD. In addition, the growth of Li2CO3 on the surface was found at 250 degrees C, which can be attributed to chemical reactions between Li2O2/Li2O and carbon-containing species (e.g. hydrocarbons) present in the XPS chamber. This finding highlights the challenges of developing stable carbon-based oxygen electrode for Li-air batteries.