The equilibrium temperature (T-0) at which the tetragonal and monoclinic phases of either ZrO2 or HfO2 coexist is generally defined by the middle temperature of A(s) (the onset transformation temperature on heating) and M-s (the onset transformation temperature on cooling). It cannot be directly determined due to the athermal nature of the martensitic transformation. Practically, the determination of T-0 is important for the prediction of A(s) and M-s in ZrO2 or HfO2-based materials. In this work, the ZrO2-HfO2 system was studied experimentally by differential thermal analysis (DTA) to obtain the martensitic tetragonal monoclinic transformation temperatures in the temperature range of 1273-1973 K. The T-0 temperatures obtained for ZrO2 and HfO2 are 1367 +/- 5 and 2052 +/- 5 K, respectively. They are adopted for the assessments of the Gibbs energy parameters of these two oxides. A reasonably calculated ZrO2-HfO2 phase diagram is presented.