The use of heteroepitaxial Si / Pr2O3 / Si(111) systems as semiconductor-insulatorsemiconductor (SIS) stacks in future applications requires a detailed structural characterization. We used X-ray reflectivity (XRR) to control layer thickness and interface roughness, standard X-ray diffraction (XRD) to analyze the Pr2O3 phase, orientation and crystal perfection, and grazing incidence XRD to study the thin epitaxial Si top layer. Transmission electron microscopy (TEM) was used to prove the results by direct imaging on a microscopic scale. Pr2O3 grows epitaxially in its hexagonal phase and (0001) orientation on Si(111) substrates. An epitaxial Si overgrowth in (111) orientation and good perfection is possible, but such Si layers exhibit two stacking twins, one with the same in-plane orientation as the substrate and one rotated by 180° around the Si  direction.