The dependence of the Si/SiO2 interface characteristics on the thickness and oxidation temperature for SiO2 films grown on different Si orientations was studied by spectroscopic immersion ellipsometry (SIE) and atomic force microscopy (AFM). Essentially, SIE uses liquids that match the refractive index of the films, thereby optically removing the films and consequently increasing the sensitivity to the interface. We show that as the thickness of the thermally grown SiO2 overlayer increases, the thickness of the suboxide layer at the interface also increases, and the average radius of the crystalline silicon protrusions (roughness) at the interface decreases for the three different Si orientations (100), (110), and (111), and two different oxidation temperatures (800 and 1000-degrees-C) studied. The dependence of the interface roughness on the thickness of the SiO2 overlayer was confirmed by AFM. The results include unintentionally and intentionally roughened Si samples and are shown to be consistent with the commonly accepted Si oxidation model.