Abstract The structural characteristics of InP single crystals have been investigated depending on the radiation effects produced by fast neutrons and the full spectrum of the reactor neutrons and subsequent heat treatment procedures. The lattice parameter of InP single crystals decreases under neutron irradiation as opposed to other III–V semiconductor compounds. Fast neutrons make the main contribution to the change of the lattice parameter. A thermal neutron component initiates the formation of Sn atoms in the material, but does not influence the change of the lattice parameter significantly. Heat treatment of the irradiated samples up to 600 °C causes annealing of the radiation defects and recovery of the lattice parameter. With increasing neutron fluences, the lattice parameter becomes even higher than before irradiation. The data analysis proves the following assumption: anti-site defects P In mainly contribute to the lattice parameter decrease during neutron irradiation of InP. In this case, anti-site imperfections produce an effect similar to that of vacancy defects.