Comparison of the annealing properties of radiation‐induced conductivity changes in GaAs indicates that about 10% of the damage created by reactor irradiations anneals in a manner quite similar to but not identical with that created by 1‐MeV electrons. The remaining neutron damage requires much higher annealing temperatures and is presumed to result from complicated damage structures characteristic of highly energetic knock‐on atoms (e.g., disordered regions). Heavy neutron irradiation of eitherp‐ orn‐type GaAs results in very high resistivities which appear to be influenced by the presence of slow surface states. Energy levels resulting from neutron irradiation are estimated to lie at approximately 0.1 and 0.5 eV below the conduction band and at 0.6 eV above the valence band. Moderate irradiation of GaAs by fast neutrons gives rise to a continuous optical absorption spectrum for wavelengths beyond the fundamental absorption edge, with the absorption increasing as the inverse square of the wavelength. Similar behavior occurs in CdTe and CdS after neutron irradiation. Although this effect is not well understood, it is suspected of being associated with defect structures characteristic of fast neutron bombardment, since heavy bombardment with electrons does not produce the same behavior.