[en] The characteristic feature of the S^- centres is the p-hole, which corresponds to the lack of one electron to the Ar configuration. Because of this simple structure these centres can be regarded as model systems for the study of defect electrons and for their theoretical treatment. The validity of calculations from first principles made for one-electron systems can be tested for defect-electron systems too. The distribution of the unpaired electron over several shells of neighbour ions was determined by ENDOR measurements in KCl. Experimental data are presented and compared with theoretical calculations