Infrared attenuated total reflection spectra from thin SiO2films sandwiched between a Ge prism and a Si substrate were investigated. The measurements were performed in the range of Si‐O‐Si stretching vibrations and compared with calculated spectra using bulk values for the SiO2dielectric function. This comparison enabled confirmation of the experimentally observed peak broadening and peak shift of the longitudinal‐optical‐phonon mode at ∼1250 cm−1for films thicker than 30 A˚ by using the exact expression for calculatingp‐polarized spectra. It is also shown that the linear approximation for vibrational spectroscopy in this frequency range is only valid for thicknesses less than 15 A˚. ©1995 American Institute of Physics.