The liquidus surface in the HfO2-Y2O3-La2O3 system was studied by differential thermal analysis in helium at temperatures of up to 2500°C, derivative thermal analysis in air at temperatures of up to 3000°C, x-ray diffraction, optical microscopy, and electron microscopy. The liquidus surface was found to comprise five primary crystallization fields-those of theH-Y2O3-,C-Y2O3-,F-HfO2-, andX-La2O3-based solid solutions and the pyrochlore phase La2Hf2O7. Three invariant equilibria were identified in the system studied-two peritectics and one eutectic.