The ZrO2‐HfO2system was investigated by metallo‐graphic, X‐ray diffraction, and microprobe analysis using are‐melted samples which had been annealed and quenched. The monoclinic‐tetragonal inversion was investigated by DTA and high‐temperature X‐ray diffraction analysis. Results reveal that zirconia and hafnia are completely miscible in all proportions in the binary system. Within experimental error, the melting points of the compositions lie roughly on a straight line connecting the melting points of oxides. There is a continuous increase in the monoclinic‐tetragonal inversion temperature as the composition moves from ZrO2to HfO2. The lattice parameters of the monoclinic phase follow smooth curves which have a negative deviation fr