High temperature X-ray diffraction analysis and thermogravimetric analysis up to 1400 degrees C and 1500 degrees C, respectively were used to examine the resulting phases and lattice parameters of praseodymium oxide in situ in N-2 and O-2 atmosphere. In N-2 (log(pO(2)/Pa)=1.5), a superlattice reflection of the C-type rare earth structure was detected between 700 degrees C and 1000 degrees C before the decomposition of the defect fluorite into A-type rare earth structure. On the other hand, In O-2 (log(pO(2)/Pa)=5.0), such a superlattice reflection was not observed. At the phase transition from defect fluorite (PrOX) to A-type rare earth structure (Pr2O3) for the powder sample on the Pt plate heater, no selective orientation occurred for the Pr2O3 in N-2 (above 900 degrees C), however, a strong c-axis orientation of Pr2O3 perpendicular to the Pt plate was observed in O-2 (above 1400 degrees C). Such results have not been previously reported. Thermogravimetric analysis helped to identify intermediate phases.