Current‐voltage measurements and radiotracer experiments have been carried out to examine the nature of the passivation mechanism responsible for the reduction of sodium ion instabilities in MOS structures prepared from oxides grown in the presence of HCl. It has been found that the ions in the passivated oxides can still migrate to the vicinity of the silicon interface, but have no effect on the surface potential because they become neutralized, probably in the interaction with a chlorine‐associated species. The exact identity of this species and the details of the charge transfer to the Si electrode remain unresolved.