Utilizing a sequence of optical models with increasing complexity, the inclusion of an interface layer between silicon and thermally grown oxide for modeling spectroscopic ellipsometric data is shown to be necessary to best interpret the data. This interface is clearly shown not to be an analysis artifact, and is important even when the optical constant spectra for silicon and thermally grown silicon dioxide are simultaneously determined. For this work, spectroscopic (0.75-6.5 eV) ellipsometric data sets taken at multiple angles of incidence from seven samples with oxide thicknesses from 2 to 350 nm were analyzed using a self-contained multi-sample technique to obtain Kramers-Kronig consistent optical constant spectra. A detailed study was made of parameter correlation effects involving the optical constants used for the interface layer. The resulting thermal silicon dioxide optical constants were shown to be independent of the precise substrate model used, and were found to be similar to 0.4% higher in index than published values for vitreous silicon dioxide. The resulting crystalline silicon optical constants are comparable to previous ellipsometric measurements.