We demonstrate the epitaxial growth of hexagonal Pr2O3(001) on Si(111), with x-ray ω-scan full width at half maximum values as low as 0.06°, which is comparable with Si substrates. We find that a phase transition takes place during the anneal of the as-grown films in N2 below the growth temperature. The annealed films display a cubic structure isomorphic to manganese oxide, (111) oriented but 180° rotated about the Si(111) surface normal. The phase transition can be due to nitrogen incorporation. © 2001 American Institute of Physics.