We have investigated, using differential infrared spectroscopy, the transition region between single-crystal silicon and its natural thermally grown oxide. By monitoring the thickness-dependent behavior of the stretching mode of the Si-O bond near 1075 cm-1, we isolate bulk oxide characteristics and features arising from interface constraints, finding that films up to 100 Å are still affected by interface effects. For thicker films we observe a consistent degree of asymmetry in the Si-O absorption band of about 9 cm-1, which does not exhibit a strong dependence on thickness. By contrast, the peak position, width, and degree of symmetry are found to be sensitively dependent upon film thickness below 100 Å, providing evidence for structurally distinct phases of silicon dioxide. Our initial interpretation suggests that the infrared spectra of such thin layers may be significantly affected by strain originating at the Si-oxide interface, rather than oxygen deficiency or overstoichiometry.