Reducing the energy of the primary ions will improve the depth resolution in SIMS depth profiling. At ultra low energies (<150 eV) the sputtering yield is drastically reduced. In this report we will focus on Cs+ ions, which are commonly used for their enhancement of negative secondary ion formation, and discuss the impact of these extreme conditions on the steady state surface concentration of cesium. In contrary to what is found from sputtering-based retention models, the steady state surface concentration of (retained) cesium on silicon reaches a maximum value in the order of 15 at%. (C) 2008 Elsevier B.V. All rights reserved.