Yttria is of interest for infrared applications because of its high wavelength cutoff (9 μm) which results in low emissivity, combined with high refractoriness (M.P. 246°C). Pore-free, single phase polycrystalline yttria is transparent since the structure is C-type cubic. An addition of 6 to 14 mole percent lanthana yields a composition which is two-phase (cubic plus hexagonal) during a high temperature sintering and transforms to a single cubic phase during a lower temperature anneal. This permits the attainment of low porosity levels by a unique transient second solid phase sintering mechanism. Sintering of discs and domes to near-net-shape has proven feasible. Room-temperature transmittance, absorption, and scattering measurements have been determined as a function of process parameters. Absorption coefficients have been lowered by a factor of five over the last year in an ONR sponsored program to 0.02 cm-1. A significant absorption peak at 3246 cm-1 was removed by controlled oxygen pressure annealing. An apparatus was developed to measure transmittance as a function of temperature and atmosphere. Increased absorption was observed above 1000°C in 02 and 1300°C in H2. Curves of transmittance as a function of oxygen partial pressure were determined. These were interpreted in terms of stoichiometry and the role of point defects on optical transmittance. An interrelationship between high temperature optical and electrical properties is discussed.