In an effort to understand observed differences in high-temperature steam corrosion resistance, high-frequency (>500 Hz) a-c impedance measurements have been used to monitor the permeability of oxidation films formed on Zircaloy-2 and Zircaloy-4 specimens exposed to high-temperature steam. Differences in oxidation resistance are related to differences in oxidation film permeability and ultimately to film characteristics. When these results are added to the existing data base established for zirconium alloy oxidation films using the high-frequency impedance technique, improved understanding of the role of bulk corrodent access to the metal-oxide interface during oxidation becomes possible. In addition, differences in film character resulting from differences in oxidation environment become apparent upon review of the existing data base. Such differences may have practical application in selecting material preconditioning procedures and suggest explanations for some oxide film memory effects reported in the literature.