The spatial variations of the transverse‐optic and longitudinal‐optic mode vibrations of the bridging oxygen asymmetric stretch have been studied in the region of the SiO2/Si interface in thermally grown oxides. Comparing the behavior of the two modes it is concluded that vibrational frequency shifts are due primarily to stoichiometry changes and not stress or densification. Fitting to and deconvolution of the measured peak lineshifts as a function of oxide thickness enables us to deduce that the region of stoichiometry variation is ≤1.6 nm.