Ultrathin Pr 2 O 3 films with various thicknesses were grown on SiO 2 /Si(100) at room temperature by evaporating Pr in an oxygen ambiance. The interfacial reactions at the Pr 2 O 3 /SiO 2 /Si interfaces were investigated in situ as a function of annealing temperature by using X-ray photoelectron spectroscopy (XPS). The intermixing between Pr 2 O 3 and SiO 2 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 Pr 2 O 3 and SiO 2 . By controlling the thickness of the ultrathin Pr 2 O 3 film deposited, structures of Pr silicate/SiO 2 /Si and Pr 2 O 3 / Pr silicate/Si can be readily achieved by ultrahigh vacuum (UHV) annealing.