Abstract The diffusion of 41Ar and 39Ar in single crystals of potassium chloride and potassiumdoped sodium salts was measured after neutron irradiation. There seems to be a relationship between the activation energy and the energy barrier for interstitial diffusion. Low diffusion coefficients were measured not only as a result of increased neutron dose but also after doping with SrCl 2. It is assumed that trapping of rare-gas occurs and that vacancy clusters act as trapping centres. The damaged lattice partly recovers during an anneal but numerous clusters remain after prolonged heat treatment at 650°.