Several epoxy- and polyimide-based fiber-reinforced materials were irradiated to neutron fluences of 4.1 × 1021 n/m2, E > 0.1 MeVat 4.2 K. Postirradiation testing of mechanical and electrical properties was performed at 77 and 300 K. Mechanical strength was observed to decrease for the epoxy-based materials while the polyimide-based materials showed essentially no change in strength. Electrical properties revealed little or no pattern of degradation at all levels of radiation exposure.