A method to probe the depth dependence of the electrical impedance of an Al2O3 film on Al metal has been developed. The deposition of high electron affinity species (O-3, NO2, Cl-2) at 90 K on the outer surface of an oxide film (d = 20-25 Angstrom) on Al(111) produces negatively charged adsorbate molecules as a result of the trap-mediated electron tunneling from the metal. The capacitor produced in this way exhibits an electrostatic field gradient across the Al2O3 film which has been depth profiled by measuring the shifts in the Al3+(2p) and O2-(1s) X-ray photoelectron spectroscopy (XPS) features originating from the film. Analysis of the XPS peak shifts and shape variation shows that most of the potential gradient exists in the inner thin layer of Al2O3 (d = 7 Angstrom) adjacent to the metal surface. We postulate that the inner oxide layer detected here is the crucial region for corrosion protection of Al.