The previously unknown experimental HfO2-Ta2O5-temperature phase diagram has been elucidated up to 3000 degrees C using a quadrupole lamp furnace and conical nozzle levitator system equipped with a CO2 laser, in conjunction with synchrotron X-ray diffraction. These in-situ techniques allowed the determination of the following: (a) liquidus, solidus, and invariant transformation temperatures as a function of composition from thermal arrest experiments, (b) determination of equilibrium phases through testing of reversibility via in-situ X-ray diffraction, and (c) molar volume measurements as a function of temperature for equilibrium phases. From these, an experimental HfO2-Ta2O5-temperature phase diagram has been constructed which is consistent with the Gibbs Phase Rule.