we have developed an experimental setup to facilitate study of the surface reactions of single-crystal silicon with Fourier transform infrared (FTIR) spectroscopy in a variety of aqueous environments. Employing a short optical path length through the silicon sample allows access to the critical low-frequency region of the spectrum (similar to 850-1500 cm(-1)) that cannot be probed with traditional multiple internal reflection (MIR) techniques. The utility of this technique is demonstrated in a study of the etching of ultrathin SiO2 on Si(100) in dilute hydrofluoric acid. This approach provides in-situ access to the SiO2/Si(100) interface that is revealed as the overlying oxide is stripped away. We find that this layer is, indeed, structurally distinct from the rest of the SiO2 film, consistent with a marked change in reactivity as etching nears the Si(100) substrate.