Abstract Specimens of beryllium which had received neutron irradiations up to an integrated fast flux of 10 22 nvt have been examined after annealing at various temperatures. For beryllium which had received the maximum irradiation, the annealing treatments have resulted in significant decreases in overall density of the beryllium specimens, the density decrease being 0.8 % and 20 % after annealing individual specimens for one hour at 595° C and 995° C respectively. The quantity of neutron-induced gases contained in the beryllium has been shown to be approximately 23 cm 3 gas per cm 3 metal for the integrated fast flux of 10 22 nvt. Analysis of the gas shows that it is mainly He 4. Metallographie evidence confirms that the density decreases are a direct result of void formation in the metal; it is concluded that the voids contain He 4 at pressures high enough to cause growth of the voids at the annealing temperatures. The detailed results of the constituent analysis of the neutron-induced gas confirm that the greater part of the gas arises from the (n, 2n) capture reaction on beryllium.