We present an EPR/ENDOR study of the so-called centres produced by x-irradiation at of NaCl:Fe crystals grown in a chlorine atmosphere which had previously been studied by means of EPR. It is shown that the centres each consist of a substitutional ion with a defect in the nearest-neighbour chlorine site. The defect, which is responsible for the very large axial zero-field splitting of the centres, is either an interstitial ion, which forms a weakly bound split interstitial with the nearby substitutional ion, or a divacancy consisting of a nearest-neighbour vacancy and a vacancy next to it on the same axis. At room temperature the centres are transformed into cubic centres, in which the ion is situated in an interstitial site.