The effect of fast‐neutron irradiation on infrared attenuation in single‐crystal GaAs has been investigated. Bothn‐ andp‐type materials of several dopant concentrations were exposed to seven fluences of fast neutrons (> 10 keV) from 5×1014to 1.5×1018neutrons/cm2. Infrared transmission was measured before and after exposure. At low fluences, the free‐carrier absorption of all samples was reduced by neutron irradiation. At the higher neutron fluences, an absorption process which showed no structure and varied approximately as the square of the photon energy was observed to dominate the free‐carrier absorption. This absorption increased linearly with fast neutron fluence and was the same for all samples studied independent of impurity type and initial concentration. The results are in agreement with a neutron‐produced spike absorption mechanism proposed by McNichols and Ginell.