Although being investigated for decades, the formation of atomic secondary ions in sputtering is still not completely understood. By briefly summarizing the available information on velocity and work function dependences of measured ionization probabilities, we show that the formation of practically all analytically useful ions is governed by a combination of non-adiabatic and substrate excitation related ionization mechanisms. We also demonstrate that the oxygen matrix effect can be well described by assuming a statistical distribution of oxygen atoms within the surface emission zone. (C) 2008 Elsevier B. V. All rights reserved.