Ultrathin Pr2O3 films with various thicknesses were grown on SiO2/Si(100) at room temperature by evaporating Pr in an oxygen ambiance. The interfacial reactions at the Pr2O3/SiO2/Si interfaces were investigated in situ as a function of annealing temperature by using X-ray photoelectron spectroscopy (XPS). The intermixing between Pr2O3 and SiO2 can be observed even at room temperature, resulting in the formation of Pr silicate at the interface. The intermixing is enhanced at elevated temperatures, leading to the continual growth of the silicate and consumption of Pr2O3 and SiO2. By controlling the thickness of the ultrathin Pr2O3 film deposited, structures of Pr silicate/SiO2/Si and Pr2O3/Pr silicate/Si can be readily achieved by ultrahigh vacuum (UHV) annealing. (C) 2004 Published by Elsevier B.V.