The effects of thermal annealing on the interface reactions and bonding structures of CeO2/La2O3 stacked dielectrics are investigated by x-ray photoelectron spectroscopy (XPS) measurements. We found that the thermal treatment can lead to significant interface oxidation and silicate formation both in the bulk and at the interface. Sample with 600 °C annealing exhibits some better interface properties than that at 500 °C. Detailed XPS studies further reveal that that a small amount of Ce3+ states were re-oxidized into Ce4+ states at the CeO2/La2O3 interface as the annealing temperature increased. This effect became more obvious at the location closer to the bulk La2O3. The phenomena and mechanisms revealed in this work provide some useful guidelines for determining better processing conditions or processing sequence for subnanometer EOT gate dielectric fabrication.