The authors present the application of synchrotron Bragg diffraction, x-ray reflectance (XRR), and x-ray photoelectron spectroscopy (XPS) to study silicon loss in the low temperature plasma oxidation of silicon-on-insulator (SOI) wafers. The Laue oscillations of the Si(004) Bragg peak provide a direct measure of the number of lattice planes that are consumed in the silicon device layer during processing, while the Fourier transform of the XRR data provides a model-independent determination of the increase in the combined thickness of the silicon and surface oxide. XPS measurements provide complementary information concerning changes in thickness, chemical composition, and the bonding of the surface oxide. These methods were applied to samples processed in an oxidizing plasma system at temperatures below 250 degrees C. The authors find that 2.7 +/- 1 angstrom of silicon, corresponding to two lattice planes, is consumed while the combined thickness increases by 2.7 +/- 0.8 angstrom, corresponding to a net increase in the oxide thickness of 5.4 +/- 1.3 angstrom. Thus, the ratio of oxide growth to silicon loss is about 2.0 +/- 0.9, somewhat lower than the bulk ratio of 2.2, but within experimental error. The XPS measurements show the increase to be 5.5 angstrom. Additionally, XPS shows clearly the consumption of silicon to form Si2O and SiO2 and the net oxidation of Si2O3 to SiO2. (C) 2008 American Vacuum Society.