X-ray photoelectron spectroscopy (XPS) and Si K-edge x-ray absorption near-edge spectroscopy (XANES) have been used to characterize 0.5-20 nm thick SiO2 films on Si(100). In XPS measurements, the chemical shift of the Si(-O) 2p peak increases with oxide thickness. However, by neutralizing the surface with low-energy electrons, and using the C 1 s as a reference, a chemical shift of 3.9 +/- 0.1 eV is found for neutralized oxide films, and the energy difference between the 0 1 s and Si(-O) 2p is independent of oxide thickness. XANES measurements confirm an invariant chemical shift with oxide thickness, the Si K edge from SiO2 being 1845.50 +/- 0.05 eV for all samples. XPS and XANES data then clearly show that any change in chemical shift of the Si(-O) 2p with oxide thickness is mainly due to a surface charging rather than microscopic strain induced by the molar volume mismatch between the oxide and the substrate, as has been previously believed.