Phase diagram of the Al2O3HfO2Er2O3 system was constructed for the first time in the range of 1600 degrees C2800 degrees C by methods of X-ray analysis at ambient temperatures, petrography, DTA in H2 at temperatures to 2300 degrees C, and electron microprobe X-ray analysis and presented as isothermal sections at 1250 degrees C and 1650 degrees C, liquidus and solidus projections, melting diagram, schematic of the reactions proceeded during sample crystallization, three vertical sections. No ternary compounds and appreciable solid solution regions on the base of components or binary compounds were found in the ternary system. The structure of the bounding systems defines the phase equilibria in the ternary system.