A simple secondary ion mass spectrometry (SIMS) apparatus has been constructed. A Superior Electronics, 3K/5U, electron gun has been converted to an ion gun capable of producing a 1 mm diam, 2 keV, 1 &mgr;A beam. To achieve wide dynamic range at high resolution in the mass analysis of the secondary ions using a quadrupole mass spectrometer (QMS), the ion injection energy must be ≲4 eV. Ions sputtered from the target with these low energies are selected with a simple cylindrical mirror analyzer placed in front of the QMS. Single monolayers are easily analyzed using a beam density of 10−7A/cm2; bulk profile analysis is performed while sputtering with higher beam densities. Signal‐to‐background ratios in excess of 106are acheved. Positive SIMS of clean and cesiated GaAs single crystals have been obtained and are discussed. A composition versus depth profile of a GaAs&sngbnd;AlxGa1−xAs sandwich structure is also presented.