Mass resolved ion scattering spectrometry has been used in combination with secondary ion mass spectrometry to study the surface composition of oxidized silicon samples (contaminated and sputter cleaned thin layers of SiO2 on Si and Si exposed to an oxygen jet). Using 1.5-3 keV Ne-20(+) ions at near-normal incidence and scattering angles around 133 degrees, the energies of Ne+ and Ne2+ after reflection from Si-28 ranged from 46 to 76 eV. Under these conditions the radius of the blocking cone experienced by escaping ions becomes very large (>0.3 nm). Therefore, any adsorbates that may be present at the surface will strongly reduce the scattering signal from target atoms. In accordance with this supposition Si atoms are initially almost undetectable on samples covered with a "natural" adsorbate layer. The scattering yields from Si atoms measured with sputter cleaned SiO2 or oxygen saturated Si amount to only 8%-10% of the yield for clean Si. These numbers are lower by more than a factor of three than what would be expected if the surface composition were reflecting the bulk atomic fraction of Si in SiO2. Apparently, sizable amount of oxygen is residing on the ion bombarded sample. The results were found to be independent of the charge state of the escaping probe ions. Hence neutralization effects do not seem to play an important role.