Multiple total internal reflection Fourier transform infrared spectroscopy is used to detect combinational phonon bands ofSiO2at 1645, 1852, and2000 cm−1in thin films produced by plasma enhanced chemical vapor deposition. The isotopic shifts of these bands in films deposited fromSiH4/18O2mixtures proved that combinations ofSiO2phonons give rise to the additional absorption peaks. Detection of these combinative phonon bands enables one to use Si multiple total internal reflection crystals for studying Si–O phonon absorptions inSiO2films on Si. In principle, films as thin as a few angstroms can be detected using the infrared absorption by the combinational bands. ©1997 American Institute of Physics.