We present here a combined study of XPS and TOFSIMS on the oxidation of Si/Si1-xGex/Si layers with x = 0.2 and top Si thickness from 0.5 to 3 nm. The oxidation conditions correspond to the formation of a 2, 4 or 6 nm thick SiO2 layer. XPS shows that, the oxidation of Ce does not occur and that the actual oxide thickness measured from the Si2p peak is in very good agreement with the expected thickness except for the thinnest top layer. The samples with the thinnest top layer show a fully oxidized cap layer and the oxidation continued through the SiGe layer to produce an oxidized layer. These results were largely confirmed by TOFSIMS depth profile measurements with a dual beam system (IONTOF IV) which show no differences comparing the four thickest cap layers. The samples with the initially very thin Si top layers show a much thicker oxide layer estimated to similar to 20 nm, and Ge segregation is also observed.