[en] Recent studies of the effects of irradiation at 5 K on organic insulators for fusion reactors have extended the irradiation dose from 2 x 10⁹ to 1 x 10¹⁰ rads and have looked for changes due to fast neutrons. For radiation conditions in this experiment the latter had little effect upon electrical and mechanical properties. At a dose of 1 x 10¹⁰ rads, particle-filled epoxies are at ''end of life'' in terms of mechanical strength, while fiberglass-cloth-filled epoxies retain sufficient strength for use. Electrical-resistivity and voltage-breakdown vaues are reduced in some materials but remain in a usable range. Two sheet-type materials show excellent stability in their electrical properties