Anodization and subsequent cathodic polarization of a thin-film sample of Zr were studied with in situ neutron reflectometry and electrochemical impedance spectroscopy (EIS). The results show the originally 485 A thick sputter-deposited film generally behaved similar to a bulk electrode in neutral Na2SO4 solution. The anodic oxide, grown by applying 0.5 V potential steps, contained a significant amount of hydrogen in one form or another. The observed anodization ratio was somewhat higher than the literature value determined by coulometry, while the Pilling Bedworth ratio was in good agreement with published data. Oxide layer thickening, accelerated immediately after an anodic polarization was applied, persisted for many hours, suggesting the migration of ions continues for an extended time. The oxide cracked when the applied potential reached 1.5 V and its thickness reached similar to 100 A, causing loss of passivation. Surprisingly, the EIS behavior of the sample with cracked oxide could still be well represented by a single time-constant equivalent circuit consisting of a capacitor and a leakage resistor, albeit with much lower resistance. This indicates that the cracked and intact regions of the electrode behave essentially independent as parallel electrodes. The proposed modified equivalent circuit can explain a number of other unusual observations, including those seen under cathodic polarization. (C) 2008 The Electrochemical Society.