The problem of charge compensation during SIMS depth profiling of resistive‐layer structures on insulating substrates is discussed with reference to BF2 and As‐implanted silicon‐on‐sapphire (SOS). Accurate positive ion fluorine profiles (from the BF2 in SOS samples) could not be obtained using electron beam charge compensation, due to an intense electron stimulated desorption (ESD) signal at mass 19. This species is shown to be F+. Evaporation of a gold‐surface‐edge electrode, 50 nm thick, represents an alternative charge compensation technique, providing a conduction path from crater edge to sample holder that bypasses the insulating substrate. Fluorine profiles can thus be obtained to within a few hundred angstroms of the sapphire. Such an electrode may be of more general use as it simplifies the setting up of electron beam charge compensation, where profiles are to be taken through a conductor and into an insulator, facilitating correct charge compensation at both sides of the interface. Accurate arsenic profiles (from the As in SOS samples) also require careful charge compensation, particularly in quadrupole based machines. Small changes in surface potential can lead to the illusion of dopant pile up (at an interface, for example) as Si2O is swept into the bandpass of the instrument. It is demonstrated that the intercomparison of mass channels, tuned to the same mass but different starting potentials, is an invaluable aid in the detection of such artifacts. The comparison of matrix‐channel‐ion images obtained at different stages in the profile is also useful and shows that the charging of the crater base does not occur uniformly.