Particle beams generated in a Kaufman‐type ion gun from CC12F2, CCl4, and BCl3 have been characterized with a mass spectrometer. The composition of ionic and neutral species emanating from the gun varies strongly with the gun operating conditions. For all three gases major components are Cl+2 and Cl+ among the ions, and Cl2 among the neutrals. Various other ions and neutral fragments of the form CClx Fy, CClx, and BClx are also observed. Changes in the gas composition versus the magnetic field in the gun and versus gas pressure are related to corresponding measurements of the etch yield of aluminum. These results, together with data on the etch yield versus ion energy and incident angle, suggest that the overall etching includes contributions from physical sputtering, direct reactive ion etching, and, especially with CCl4 and to some extent with BCl3, from chemical sputtering. The implications of the beam diagnostics for other experiments, etching such materials as III‐V compounds, silicon, and silicides are also discussed.