Europium (Eu) was found to act as a solid-state sintering aid in Y2O3 optical ceramics by controlling ionic diffusivity, which in turn leads to enhanced optical transparency. Transparent ceramic samples of Eu-doped Y2O3, with no additional additives, were sintered by uniaxial vacuum hot pressing under 40 MPa and maximum temperature of 1580°C. Optical attenuation was found to decrease with increasing Eu concentrations between 0 and 5 at% for ceramics processed under the same sintering conditions. In order to study the effect of Eu concentration on ceramic densification, the strain rate and grain size during sintering at constant temperature and varied pressure were measured. A diffusional flow densification model was used to derive instantaneous effective diffusion constants for the densification process. Diffusion constants were found to increase with increasing Eu concentration according to a log–linear relationship. Eu2+ was detected in samples after hot pressing through fluorescence spectroscopy, and the extrinsic defect chemistry was found to be dominated by the reduced Eu in solid solution with Y2O3. A sintering model with diffusion rate limited by yttrium interstitial transport and controlled by the incorporation of Eu2+ onto the cation sublattice was found to be in good agreement with experimental diffusivity data.