The effects of fast neutrons on four commercial poly crystalline alumina products of high density were investigated. These materials have been considered for use as electrical insulators in nuclear-powered thermionic converters. Fast-neutron fluences up to 8.4 × 1021 n/cm2(>0.1 MeV) were achieved; irradiation temperatures ranged from 60 to 1230°C. Neutron damage was manifested by gross fracturing, volume increase, and separation at grain boundaries. There was very little damage at temperatures below 100°C, but it was much greater at temperatures from 570 to 1070°C (the temperature range for thermionic applications). At 1100°C and above, in-reactor thermal annealing was rapid enough to reduce damage effects significantly, but increased damage occurred to specimens irradiated at much higher temperatures (∼1230°). Based on the irradiation results, the following conclusions and recommendations were made for the use of poly crystalline alumina in thermionic converters: (a) the alumina should be of high purity to minimize gross fracturing, and it should be of small grain size to minimize the effects due to separation at grain boundaries; (b) the thermionic device should be designed for continuous operation because thermal cycling apparently promotes separation at grain boundaries; (c) the design should allow for an increase in volume of the alumina insulators of about 3%; and (d) alumina of high purity and small grain size can withstand fast-neutron fluences up to about 4.3 × 1021 n/cm2 in an ETR-type neutron spectrum and up to about 2.8 × 1021 n/cm2 in an EBR-II-type spectrum for those neutrons with energies >0.1 MeV.