Thermal neutron irradiation produces spatial and energetical homogenously distributed defects in n-GaAs which act as charge carrier traps and scattering centres and as recombination sites for photogenerated charge carriers. Therefore, after neutron irradiation the charge carrier concentration and mobility and the maximum photocurrent are lowered and the photocurrent onset potential is further shifted in positive direction versus flatband potential. Annealing of the irradiated samples leads to defect annihilation and thus, to the recovery of the original electrochemical characteristics. Annealing at temperatures above 600°C leads to an arsenic deficient surface with electronic defect states in the band gap in the near-surface region. Recombination at these defects only influences the maximum photocurrent, but not the photocurrent onset potential.