Thin heteroepitaxial praseodymia films with fluorite structure on Si(111) were annealed under ultra-high vacuum conditions at temperatures in the region of 100 up to 300 °C. Afterward investigations by x-ray diffraction, grazing incidence x-ray diffraction and x-ray reflectometry were performed to obtain information about structural changes of the film during the annealing process. For this reason, praseodymia Bragg peaks were carefully analyzed within the kinematic diffraction theory. This analysis demonstrates the coexistence of different praseodymia phases depending on the conditions of preparation. Here, annealing of the samples up to 150 °C leads to a homogeneous film with a PrO(1.833) phase and with negligible strain since both the lateral and vertical lattice parameters nearly match the corresponding bulk praseodymia phase. Further annealing leads to oxygen loss accompanied by significantly increased lattice parameters. Since the lateral lattice parameter is pinned at the interface, the vertical lattice constant has to increase considerably due to the tetragonal distortion of the film. This causes the decomposition of the film into two oxide species with significantly different oxygen contents. Annealing at 300 °C reduces the film almost completely to PrO(1.5) which has the minimum content of oxygen.