Electrochemical oscillations at silicon electrodes anodized in hydrofluoric acid are a well known but poorly understood phenomena. It is less well known that potential oscillations are also observable during anodic oxidation in fluoride-free electrolytes. In situ measurements of stress, as well as x-ray reflectometry, atomic force microscopy, and ellipsometry used to investigate the properties of the thin anodic oxide which covers the electrode surface:during the oscillations. The results indicate a transition in the oxide morphology a thickness of about 10 nm. Based on this structural transition a model is developed which explains all observed features of the oscillation process in a consistent way.