GaAs and GaP thin film and bulk samples have been irradiated with neutron fluences of 1018nvt and the effect on the optical absorption edge and free carrier region determined. In the case of both GaAs and GaP films, which exhibit bulk or near bulk optical properties before irradiation, large optical absorption edge shifts of ∼0.3 and ∼1.5 eV, respectively, are caused by the neutron bombardment. These optical absorption edge shifts can be ``annealed out'' by high‐temperature vacuum annealing; in the case of the GaAs films, isochronal annealing points to a Frenkel‐pair annihilation mechanism. Bulk samples of GaAs and GaP, similarly irradiated, exhibit smaller optical edge shifts than the films with substantially different quantitative behavior. High‐temperature vacuum annealing of the bulk samples also results in a recovery of normal optical properties.