Grazing incidence X‐ray reflectivity (GIXR), ellipsometry and X‐ray photoelectron spectroscopy (XPS) have been used for measuring the thickness of ultrathin SiO2films on Si(100). SiO2films were fabricated at a constant temperature to obtain a fixed interface structure for films with different thicknesses. The thicknesses obtained by GIXR and ellipsometry were in good agreement with each other, however, ellipsometry showed slightly larger values. The thickness of the adsorbed overlayer was also compared using GIXR and XPS. Uncertainties included in the XPS measurements of the carbonaceous layer thickness were estimated. The thicknesses of the carbonaceous layer obtained by XPS were slightly smaller, by about 0.16 nm, than those of the adsorbed overlayer obtained by GIXR. About 0.3–0.4 monolayer of adsorbed water molecules is believed to account for the differences in overlayer thicknesses between the GIXR and XPS measurments. © 2003 American Institute of Physics