To study the oxygen dependence of ionisation processes during O-2(+) bombardment, low-energy ion scattering was used as a surface analysis technique since it has very high surface sensitivity. An LEIS instrument was reconfigured to collect secondary ions simultaneously with the compositional analysis based on LEIS. The gradual conversion of the Si surface into an oxidised surface under oxygen bombardment has been studied. A correlation between the Si+ intensity and the oxygen concentration) could be established, i.e. I(Si+) proportional to (C-o/C-si)(2.9) It was observed that, at the stage where the oxygen concentration no longer increases, the amount of visible (for LEIS) Si atoms decreases further, implying a further structural change. This change coincides with the steep increase in the Si+ ion intensity near complete oxidation and can be interpreted as the main cause for this change in ionisation probability. (C) 2002 Elsevier Science B.V. All rights reserved.