The electrochemical behavior of aluminum during cathodic polarization was investigated with the quartz crystal microbalance, to identify changes in the electrical conduction properties of the surface film, which result in strongly enhanced electrochemical reaction rates. As a consequence of cathodic charging at potentials more negative than about - 1.45 V vs. NHE in 0.1M HCI solution, the surface film transforms from a high field electrical conductor to an ohmic conductor, and then begins to grow. The critical potential for forming this ohmically conducting film agrees with the potential below which aluminum hydroxide is expected to be more stable than aluminum oxide, near the metal/film interface. The conductivity of the cathodic film is within an order of magnitude of the proton conductivity of bulk hydrated aluminum hydroxide, Al(OH)3 - H2O. When the potential is stepped above the open-circuit potential subsequent to cathodic charging, there is a characteristic current decay during several seconds, after which the conductivity is three orders of magnitude smaller than at the cathodic potential. A mechanism is given, based on calculated overpotentials for interfacial reaction and transport processes, through which the oxide film transforms to hydroxide at cathodic potentials.