A beam of positive ions frequently derived from He or Ar is directed at the surface. Some of these ions are reflected with the loss of energy appropriate to the simple binary elastic collision of the ion beam with a particular surface atom. At any fixed scattering angle, defined by the angle between the ion source and the analyser, the energy loss of the ion is dependent only on the mass of the surface atom causing the scattering. An ISS spectrum is easily obtained by recording the number of scattered primary ions collected per second as a function of their energy from zero to the energy of the primary beam. The technique is uniquely sensitive to the outermost layer of the surface and is complementary to SIMS (secondary ion mass spectrometry).