In the far anodic regime, the anodic dissolution of silicon exhibits a resonant behavior, which results from an oscillating behavior on the microscopic scale. The interface has been investigated using in situ infrared spectroscopy in the strong oscillating regime. The interface oxide film varies in thickness by up to a factor of two, and the oxide quality, as determined from a fine analysis of the infrared polarized spectra, is also found to vary periodically during the oscillation. The relative phase of these periodic variations suggests that the origin of the oscillation mechanism lies in a periodic breakdown of the blocking properties of the oxide, which may be initiated by ion permeation in the outer part of the film. The study of oxide dissolution kinetics evidences a stratified structure of the oxide, with layers of "soft" and "strong" oxide. The fast dissolution of the soft oxide is associated with the periodic drop of the oxide thickness.