An experimental investigation was made of the impedance, rectification, and electroluminescence of anodic oxide films formed on aluminum, in an electrolyte of boric acid and ammonium hydroxide. These properties were studied, where appropriate, as a function of the surface condition before anodization, the formation voltage, the conductivity of the electrolyte, and the frequency. A constant-voltage formation technique was used. The interpretation of the results suggests that the oxide itself is a good dielectric, but that the film contains defects whose number and form depend on the conditions of formation. The defects appear to control the rectification and electroluminescence and have an appreciable influence on the impedance. The frequency dependence of the capacitance was much smaller than that found by previous workers although there was an appreciable contribution arising from geometrical effects in the electrolytic cell. The zero-thickness potential for the film was found to be in agreement with a theoretical estimate.