The determination of the oxygen concentration at the surface is the critical parameter in all models [Surf. Sci. 112(1981) 168;Nucl. Instrum.Meth. Phys. Res. B 14(1986)403; in: A. Benninghoven, B. Hagerihoff, H.W.Werner(Eds.), SIMS, Vol. X, Wiley, New York, 1997,203 pp.; Phys. Rev. B 50 (1994) 15015; in: A. Benninghoven, P. Bertrand, H.N. Migeon, H.W. Werner (Eds.), SIMS, Vol. XII Elsevier, Amsterdam, 2000,151 pp. J. Appl. Phys. 90 (2001) 4456] that study the large enhancement of the ionisation probability of positive ions by the presence of oxygen at the surface. Different analysis techniques (low energy ion scattering, Rutherford back scattering spectrometry and secondary ion mass spectrometry) are combined in order to clarify the oxygen dependence of the ionisation probability of Si+, alpha(Si)(+) during O-2(+) bombardment. A ratio of ion signals is correlated with the surface oxygen content, determined by LEIS, allowing us to study the enhancement of the ionisation probability of Si by oxygen at the surface for energies between 1 and 5 keVperO(2)(+) ion. For a wide range of oxidation conditions the oxygen enhancement can be described by a power law dependence on the oxygen content alpha(Si)(+) alpha(C-O/C-Si)(3.24), whereby the power shows no significant energy dependence for the studied energy range. The steep enhancement of the ionisation probability at high oxygen content, which is energy dependent, can be correlated to a change in the surface structure: enhanced shadowing of silicon by oxygen, observed by LEIS. (C) 2002 Elsevier Science B.V. All rights reserved.