Thermally grown SiO2 layers on Si (100) substrate have been subjected to different external voltage bias during XPS analysis to induce changes in the measured binding energy difference between Si4+ and Si0 in Si2p and SiKLL regions. The Si2pp binding energy difference increases from 3.2 to 4.8 for samples containing 1-7 nm oxide thickness, and furthermore, this difference can be influenced by application of an external bias to the sample. Application of negative d.c. bias increases the binding energy difference, whereas positive bias decreases it. The voltage dependence of the binding energy difference exhibits a sigmoid character with an abrupt change near 0 V. Both the binding energy difference and differential change between the positive and negative bias have similar functional dependence on the thickness. This is attributed to differential charging between the silicon oxide layer and silicon substrate, which is decreased when a positive bias is applied to the sample (and therefore attracting a larger proportion of the stray electrons from the vacuum chamber to partially neutralize the oxide). Similarly, when negative bias is applied, the stray electrons are repelled from the sample resulting in less neutralization and an increased differential charging. Through external biasing, it is determined that charging in the SiO2/Si system persists all of the way down to 1 nm. Application of a.c. (square-wave) bias is equivalent to simultaneous application of negative and positive bias together. However, the differential change in the binding energy difference in the positive and negative cycle is frequency dependent and approaches to the d.c. results at lower frequencies.