Using a 3 keVO(2)-beam in an in situ SIMS/RBS set-up, changes in sputter yield and ionization probability were measured in order to come to a better understanding of the behavior of the Si+ signal during the buildup of the altered layer. Based on the RBS-spectra the evolution of the sputter yield within the transient region can be determined. When combined with complementary results from a low energy ion scattering (LEIS) experiment [T. Janssens, C. Huyghebaert, W. Vandervorst, A. Gildenpfennig, H.H. Brongersma, in these proceedings], the ionization probability enhancement as a function of the oxygen concentration at the surface can be calculated. (C) 2002 Elsevier Science B.V. All rights reserved.