X-ray photoelectron spectroscopy (XPS) was performed to study the formation process of Pr2O3/ Si(001) interfaces and films during epitaxial growth and postgrowth annealing. A significant shift in the Pr and O core-level binding energy was found accompanied by an analogous shift in the Pr2O3 valence band maximum. This shift depends on the oxide layer thickness and interface structure, as indicated by ab initio pseudopotential calculations. It is caused by interface dipoles in the Si/Pr 2O3 interface region due to the accumulation of oxygen. Besides providing a insight into the growth mechanism and interface properties of high-K dielectrics on Si, our results also demonstrate the usefulness of in vacuo XPS for investigating epitaxial growth processes. © 2002 American Institute of Physics.