In secondary ion mass spectrometry (SIMS) studies it was found that analysed ions emerging from a quadrupole filter may be accompanied by a surprisingly large number of tertiary ions of the opposite charge. In the mass spectrum the tertiary ions show up at the high-mass edge of the stability diagram (x-stability limit). The peak intensity ratios of tertiary to secondary ions, i- / I+ and i+ / I-, observed with mass-filtered atomic and molecular ions, range from about 10-5 to 4 × 10-3. The effect was studied in detail as a function of the d.c.-to-a.c. voltage ratio, the orientation of the d.c. field, the frequency of operation, the ion energy, the beam current and the chamber pressure. The experimental data are consistent with the idea that most of the tertiary ions are generated under the impact of radially accelerated ions striking the quadrupole rods. Only those tertiary ions which are produced near the exit of the quadrupole have a chance to get to the detector. In the case of very light secondary ions such as H+ charge changing collisions may be largely responsible for the generation of ions of the opposite charge state.