It is of vital importance for the study of phase diagrams to investigate in situ diffusionless phase transition points at high temperatures. We have developed high-temperature techniques to measure in situ neutron and synchrotron X-ray powder diffractometry profiles. This paper reviews our recent studies obtained using these methods. The new furnace for high-temperature neutron diffraction study yields no extra peaks caused by heaters and refractory; thus, we have been able to measure very small neutron signals from the sample. Quality neutron diffraction data at high temperatures up to similar to1590degreesC have been analyzed using the Rietveld method for various materials, such as zirconia solid solutions and calcium titanate. High-resolution synchrotron X-ray diffractometry enables exact determination of the phase transition temperatures of perovskite-related materials.