The use of a helium probe technique to determine the number of single : acancies in the damage state produced by ion bombardment is described. The technique is applied to (2–7) keV Ne on Ni(100) and to 5 keV Kr on W(100). In particular it is found that after a 5 keV Ne ion bombardment (8 × 1010/cm2) on Ni(100) there remains approximately 1 vacancy/ion in the damaged state whereas after a 5 keV Kr ion bombardment on W(100) there remains approximately (0.8–1.1) vacancies/ion.