Optical constants [n.k] of 30-300 nm SiO2 films, thermally grown on c-Si substrates, are obtained numerically from [R,T] data. The calculations are done at each wavelength, no a priori assumptions regarding dispersion relations are needed. The technique also provides the film thickness, whose values agree with ellipsometric measurements. The optical constants are compared to those of bulk fused silica glass (FSG) and are found to differ in subtle but important ways. The principal absorption band dui: to the Si-O TO stretching vibrations, and measured by the epsilon " (imaginary dielectric) band, is slightly greater in magnitude and 12 cm-l narrower for the film than for the bulk material. We interpret this to indicate a simpler strain field within the film compared to the bulk material. We find, however, that the integrated intensities are identical, indicating similar densities. We show that the shift in peak position of the absorbance band with changes in film thickness is almost entirely an optical thin film effect. The absorbance band's character gradually changes from resembling the epsilon " band for very thin samples (less than or equal to 30 nm) to resembling the corresponding [k] band for thicker samples. Although epsilon " and [k] are intimately related, their positions and widths differ, thus accounting for the systematic changes in width and position of the absorbance band. Only the 30 nm sample appears to differ in any significant way from thicker samples. This difference is mainly in the low magnitude of epsilon " (indicating low density for that sample). There is a modest 2 cm(-1) shift in the peak position of epsilon ".