Distortions which occur in secondary ion mass spectrometry (SIMS) concentration profiles have been studied with samples of Si(Bi) calibrated by Rutherford Backscattering. It is shown that with normal incidence oxygen ion bombardment over the energy range 1–6 keV per atom, the SIMS profile is broadened due to ion beam matrix effects. The true profile shape can be extracted from the energy dependence of the profile width. Profile broadening has also been observed with Si(In) and Si(Sb) but not with Si(B). The observed broadening cannot be explained simply by collisional mixing of the target atoms.