The ac electrical behavior of the polycrystalline yttria was evaluated in the temperature range of 800–1300 °C in air as a function of frequency (5 Hz≤f≤13 MHz). Resistance‐temperature and resistance‐time (aging) characteristics were examined using immittance measurements and electron microscopy to establish microstructure‐property relationships. The ac electrical data indicated two distinct relaxations when analyzed in the impedance plane. These relaxations are attributed to the lumped grain and grain‐boundary contributions in conjunction with a polarization effect at the electrode/sample interface. The admittance plane analysis revealed a semicircular relaxation in the low‐frequency region, indicative of a trapping effect associated with grain‐boundaries and the electrode/sample interface. The variation in the total electrical resistance with time is found to be dependent on the starting microstructure of the sample. A sample with a larger grain size shows a smaller degree of aging at elevated temperatures. Immittance measurements suggest that the major contribution to the aging behavior comes from the evolution in the microstructure. ©1995 American Institute of Physics.