Spectroscopic ellipsometric data from 1.5–5.8 eV has been analyzed to determine the in situ optical response of the interface between Si and its thermally grown oxide. Results for , , and sample orientations show an interface of width consisting of atomically mixed Si and O of average stoichiometry . The optical data are not consistent with either microroughness at the interface or an abrupt transition between crystalline Si and or a significant accumulation of amorphous Si at the interface, but rather support a gradual transition region. The thickness and the average λ5461 refractive index of this region are in agreement with the fixed‐wavelength results, and , of Taft and Cordes. The oxide on the surface is shown to have a density about 1.2% less than that of corresponding oxides on and surfaces.