Grain-boundary mobilities and grain growth activation energies were measured on high-purity and dense yttria samples annealed in both air and 5%H2–N2 at temperatures ranging from 1200° to 1900°C. The grain-boundary mobility for samples annealed in 5%H2–N2 was higher in comparison with air-annealed materials. The measured grain growth activation energy for samples annealed in air was 356 ± 35 kJ/mol. For samples annealed in 5%H2–N2, a transition temperature of 1579°C was identified where the grain growth activation energy changed dramatically. The measured activation energies were 373 ± 60 and 770 ± 2 kJ/mol for samples annealed at temperatures below and above 1579°C, respectively. The increase in activation energy was tentatively attributed to a reduction-induced disordering of the grain boundary corresponding to the temperature at which yttrium metal liquid forms at low oxygen pressures. The data represent the best attempt to measure the near-intrinsic grain-boundary mobility for high-purity highly dense yttria. High-resolution transmission electron microscopyand high-angle annular dark field-scanning transmission electron microscopy confirmed that the grain boundaries were clean and free of any observable second phases.