A resonant cavity technique was used to measure the dielectric constant and loss tangent of ceramic insulators at a frequency near 100 MHz during pulsed fission reactor irradiation near room temperature. Tests were performed on single crystal and several different grades of polycrystalline Al2O3, MgAl2O4, AlN, and Si3N4. Lead shielding experiments were performed for some of the irradiations in order to examine the importance of gamma ray versus neutron irradiation effects. With the exception of AlN, the dielectric constant of all of the ceramics decreased slightly (<0.2% change) during the pulsed fission reactor irradiation. The dielectric constant of AlN was observed to slightly increase during irradiation. Significant transient increases in the loss tangent to values as high as 6×10−3occurred during pulsed reactor irradiation with peak ionizing and displacements per atom (dpa) radiation fields of 4.2×104Gy/s and 2.4×10−6dpa/s, respectively. The loss tangent measured during irradiation for the different ceramics did not show any correlation with the preirradiation or postirradiation values. Analysis of the results indicates that the transient increases in loss tangent are due to radiation induced increases in the electrical conductivity. The loss tangent increases were proportional to the ionizing dose rate in all materials except for AlN, which exhibited a dose rate exponent of ∼1.6. ©1996 American Institute of Physics.