AbstractA Tian-Calvet microcalorimeter capable of operating to 1000°C has been used to determine the heats of mixing of the cubic phase in ZrO2–CaO, Zr O2–MgO, and Zr O2–CaO–MgO, and ZrO2–CaO–MgO systems as well as the heat offormation of Ca6Zr19O44compound. A power supply voltage monitoring technique and a method of determining the calibration constant developed in the work enable good experimental precision to be obtained using a single cell. The drop calorimeter was also used to measure the enthalpy difference of the samples between 298 and 1100 K.MST/3379