Relative transmittance spectra of SiO on Si(111) are shown for increasing film thicknesses up to 57 nm. The thickness change between two shown spectra is about 6.3 nm. The strong peak at 983 cm^-1 is attributed to the asymmetric Si-O stretching vibration (Cachard et al. 1971). For such film thicknesses, there is clearly no shift in that peak. SiO oscillator positions are indicated by dashed lines.