Secondary ion mass spectrometry (SIMS) has been used in conjunction with isotopic labeling to determine the extent and rate of passive film hydration on aluminum. The rates at which oxygen- and hydrogen-containing species migrate through the film have been determined as a function of temperature and applied potential (cathodic and anodic polarization). The results suggest that defects such as hydroxide ions are prevalent and mobile in the oxide film, influencinc, the kinetics and mechanisms of corrosion and pitting processes.