The infra-red (IR) transmission spectra of SiO x (x ≤ 2) layers containing crystalline or amorphous Si nanoparticles deposited on p-Si substrates is simulated in the range 300–1500 cm − 1 . To that purpose the average dielectric function of the nanocomposites is calculated by means of the Bruggeman effective medium approximation. The IR spectra of the system (film and substrate) are computed. The results are compared with experimental IR spectra measured on Si–SiO x nanocomposite layers with identical composition, fabricated by thermal evaporation of SiO in vacuum followed by thermal annealing at 700 °C or 1030 °C. A good correspondence between theory and experiment is found from where valuable information about important characteristics of the investigated nanocomposites is obtained, such as matrix density, homogeneity and composition of the layers.