A dielectric test system for measuring the dielectric properties of materials under vacuum, reactor radiation, and cryogenic conditions is described. The cryovacuum dielectric tester consists of eight cylindrical test cells mounted on two rectangular, hollow cryopanels. Liquid nitrogen supplied to the cryopanels provides the cryotemperature environment. The test cells are designed to hold specimens 4.5 in. in diameter and up to 0.25 in. in thickness. Two cryovacuum-irradiation tests have been conducted. The specimens were exposed to about 1 × 10¹⁰ ergs/g (C) of gamma radiation and 3 × 10¹⁵ n/cm² of fast neutron (E > 1 Mev) radiation, vacuum in the 10⁻⁶ torr region, and temperatures of +25 C to −190 C. Capacitance, dissipation factor, and volume resistivity were determined for speciments of vinylidene fluoride, melamine resin, polycarbonate plastic, and silicone resin. A table summarizing the test results is presented.