Cathodic polarization of aluminum in acid solution produces a surface film which was studied using infrared reflectance spectroscopy (IRS), and quartz crystal microbalance (QCM), and capacitance measurements. According to the QCM results, deposition of the film began after passage of 7.2 mC/cm2 of cathodic charge at a potential of −2.0 V. This charge was consistent with IRS and capacitance measurements. The film grew at an approximately constant rate with time, indicating that its ionic conduction resistance is small. Also, the linear increase of the reciprocal capacitance as a function of film mass is consistent with film growth occurring uniformly across the electrode surface. IRS showed that the cathodic film is an amorphous aluminum hydroxide or oxyhydroxide which contains absorbed water; QCM stripping measurements found that there was at least one water molecule per aluminum ion. This extensive hydration is perhaps related to the relatively low ion transport resistance. Mass transport calculations indicated that the film was formed by direct electrochemical growth and not by precipitation.