Ellipsometric results may be inaccurate for the measured thickness of ultrathin oxide films on silicon because of the apparent refractive index changes with thickness. We have assessed this problem by comparing results on oxide thickness measured by ellipsometry with results of measurements by two independent techniques, such as medium energy ion scattering spectroscopy and high-resolution transmission electron microscopy, which should not be subject to error. The results show that appropriate ellipsometric models can provide thickness information consistent with two independent techniques, which improves the reliability of ellipsometric analysis in the nm range.