The properties of air-grown and electrochemically thickened oxide films on Zr-2.5Nb electrodes in neutral solution have been probed using dc polarization measurements, electrochemical impedance spectroscopy, and in situ neutron reflectometry. These experiments have shown that the oxide film on Zr-2.5Nb is less protective than that occurring on Ti under conditions of anodic polarization, but it is more resistant to hydrogen penetration under cathodic conditions than the oxide formed on Ti electrodes in neutral aqueous solution.