The compositional transition layer in SiO2/Si(1 0 0) interface is studied with high-resolution Rutherford backscattering spectroscopy (HRBS). The HRBS spectra of thin SiO2 films on Si, which are prepared by pyrogenic oxidation, are measured at various incident angles. The spectrum shows a sharp Si surface edge and a relatively broad edge corresponding to the SiO2/Si interface. The thickness of the compositional transition layer in the SiO2/Si interface and the energy loss straggling are determined simultaneously from the observed spectra. The obtained thickness of the transition layer is 0.53 +/-0.08 nm. The obtained energy loss straggling agrees with the empirical formula given by Yang et al. (C) 2003 Elsevier Science B.V. All rights reserved.