Polycrystalline, terbium-doped yttrium oxide samples were sintered under various conditions and examined using positron lifetime spectroscopy and cathodoluminescence. Rapid grain growth was found to occur at sintering temperatures between 1500 and 1600 degrees C, as shown by both the positron lifetimes and the luminescent intensities. The correlation between the results demonstrates the utility of positron lifetime measurements for investigating microstructural properties of materials and the importance of these properties to the luminescence.