Specimens of the compound Bi2Te2Se were prepared, in the form of pressed powder briquettes and were thermally treated to attain the ordered and the disordered states. These specimens were then subjected to irradiation with 7·5Mev protons in cumulative doses to a maximum integrated dose of 2 × 1016 protons/cm2. After each irradiation the specimens were examined by x-ray diffraction with monochromated cobalt Kα radiation. In initially ordered specimens, irradiation with protons reduced the lattice order and caused disorder in all three sublattices occupied by bismuth, selenium and tellurium atoms. This disordering is different from thermal disordering, which is confined to the Se and the Te sublattices. Irradiation of specimens with initial thermal disorder produced some lattice order and caused disorder in the Bi sublattice. The final states of order in both the initially ordered and the initially disordered specimens were remarkably similar. The results are interpreted in terms of atomic displacements, movements of defects and thermal spikes. © 1972, Taylor & Francis Group, LLC. All rights reserved.