The longitudinal optic vibrational mode associated with the asymmetric stretch of the bridging oxygen atom has been studied in amorphous SiO2 grown by plasma assisted oxidation of Si. In films ≤70 nm thick the mode is observed displaced by −6 cm−1 with respect to the bulk value (1256 cm−1). Deconvolution of the spectra combined with chemical etchback of the films shows that they are inhomogeneous, the peak shift increasing to −12 cm−1 near the Si/SiO2 interface in films thicker than ∼20 nm.