This paper presents a study of the optical properties of thermally oxidized nanoporous silicon in the wavenumber range 5000-400 cm^-1. It is shown for the first time that thermally oxidized nanoporous silicon has two broad regions of reduced transmittance in the wavenumber range from 3720 to 3000 cm^-1 and five narrow zones that have their minima at the following wave numbers: 2920, 2240, 1680, 840, and 600 cm^-1. The transmittance spectra of the thermally oxidized SiO₂-Si structure are measured in the wavenumber range 5000-400 cm^-1. It is established that the thermally oxidized SiO₂-Si structure is transparent in the wavenumber range 5000-1200 cm^-1 and has narrow absorption peaks insignificant in magnitude at 1090 cm^-1 and 620 cm^-1. It is shown that thermally oxidized nanoporous silicon can be used as a selectively absorbing and recording medium in the IR range.